I think that much of the "online left" falls into ultraleft/sectarian or reformist/opportunist camps. Oftentimes individuals will find themselves rapidly oscillating between the two. On the surface this might seem contradictory, but hear me out.
Marxism is a materialist philosophy. In order to understand any given situation, we not only analyze the reciprocal relations of all the factors in the situation, but we also have to understand the historical development of that situation, and how that has changed over time to give rise to this situation. You need to balance the particular situation, and the general situation. You can use the two, and the relationship between the two, to develop a deeper understanding of everything that is happening. This is the power of Dialectical Materialism - you gain this ability to cut through the flurry of immediate events and gain a deeper understanding of a given event in life. I'm making it sound like a super-power, but as I explained earlier, it's just looking at events through how they happened and how they're connected. It's simple, really. Where political disagreements arise is in this kind of analysis. Everyone is more or less a rational actor, and so a lot of political disagreements come down to a difference in how people have interpreted various events.
Ultraleftism and Opportunism are similar in that they are natural conclusions that people fall into when they isolate a current situation away from the complexity of factors that make up the situation into one or two factors, and really focus in on those. These factors might be the most important in a situation, but it can lead to the individual viewing these factors as absolute and immutable. That's where you get Ultraleftism. To understand it though, I'm gonna talk a bit about Reformism.
Reformism came about in the decades leading up to the First World War in large part because of the intense pace of development of Capitalism. Standards of living among the proletariat were improving and bourgeois democracy was stable, and thus the Second International was bent in a very Reformist direction. The reformists of that era viewed the trend that they existed within as a permanent feature of Capitalism - Marx wasn't around to correct them, and thus he must've missed something. This, of course, led to philosophical stagnation, and today the parties that come from that tradition don't even consider themselves Marxist.
A lot of Ultraleftism comes from the period after the First World War. Where Reformism came around in the era in which Capitalism was ascending, in the period between the World Wars, Capitalism was in severe decline. Standards of living were falling and bourgeois democracy was unstable and in the middle of a massive collapse. In this period, we can see that the political struggle in the world boiled down to 'Socialism or Fascism'. In a sense, we can see that even as far back as 1917 - Kerensky's government was weak and in danger of imminent collapse, and the two alternatives for Russia were the Bolsheviks and Kornilov - Socialism and (proto-) Fascism respectively.
Looking at all of these events occurring around them, the Ultraleftists concluded their analysis of events - that the situation was 'Socialism or Fascism' - just as they should've started their analysis. They are applying dogmatic theory to reality without understanding the underlying mechanisms within anything they're looking at. And boiling down the struggle to 'Socialism or Fascism' in the 1930s didn't work a lot of the time, because the working class broadly didn't understand the knife's edge that Capitalism was walking on in that period.
The Bolsheviks didn't adopt an Ultraleft position with regards to Kornilov vs Kerensky. The Ultraleft position would be to side with neither and attack both. But Lenin and the Bolsheviks didn't do that. They sided with Kerensky against Kornilov, but did not compromise on their position. They didn't take any kind of ownership over the policy of Kerensky while simultaneously defeating Kornilov. They denounced Kerensky for creating the conditions that allowed for the Kornilov Affair to occur in the first place. As a result, the Bolsheviks were able to maneuver themselves through the crisis of March 1917, and exploded in popularity, ultimately paving the way for their victory in October of that year. They turned the theoretical position of 'Bolshevism or Counter-Revolution' ('Socialism or Fascism') into a material reality.
In short, Ultraleftism is the dogmatic application of theory without understanding the material circumstances that lead to the development of those circumstances, or the way in which those circumstances are bound by various factors.
This stick can be bent too far in the other direction - that being Opportunism or Reformism. If your perspective is too narrow, you can also fall into the trap of assuming that the obstacles that exist for a revolutionary movement are immutable or omnipotent. This can lead to Reformism - believing that a revolutionary movement of the working class is impossible, and thus incremental economic demands are the only way forward - or Opportunism - compromising core Marxist principles to achieve limited aims in lieu of building a revolutionary movement (class collaborationism is a really common Opportunist mistake).
This answer is rather vague. "Ultraleftism is when you do Marxism wrong" isn't very helpful in terms of being a guide to action. Marxism is a science, but an inexact science. That's why debate and discussion are so important. And that's why theory is only half of it - you need to apply your theory to reality in order to determine if it's accurate or not. Test it against something, and if it's wrong - then it won't work. If it's right, it will. But you need to be able to analyze it correctly, and draw the correct conclusions from it. That's a tricky business, and that's part of the reason why we're stuck in the mess we are now.
I don't think Trotsky's permanent revolution relates to what you're calling an Ultra.
Trotsky's permanent revolution is explaining the process in which less developed nations are prevented from undergoing a bourgeois democratic revolution due to the pre-existing imperialist powers and this creates a 'permanent' revolution to persist within a nation, spurring the proletariat and peasantry to demand socialist aims in order to achieve any meaningful bourgeois-democratic demands.
Trotsky uses the February Revolution necessitating an October Revolution as an example of this exact process, and of course he was murdered before any other workers' states were created for him to analyze.