I have run into multiple really big bugs that all seem to point back to the enabling and disabling of code blocks.
1 - I saw the Demon stop working after the air attack. I was unable to really debug why because of the other following errors that made it so getting to this point was inconsistent.
2 - I have had the knight lose the ability to attack or turn around when walking into the boss room. This was fixed by disabling the action that disabled the Knight Controls group. When you run the cut scene of the knight walking into the door, the Knight Controls get disabled and sometimes don't get reenabled.
3 - When fighting the boss, quite often, the entire game would freeze. I haven't been able to figure out why.
In the image below, I have marked events that deactivate code groups but don't have a corresponding activate event.
Also, I noticed you have 2 copies of the Boss Events. Both are nearly identical. Duplicating code is always a bad idea. If you need to make changes in 1, you then have to remember to change the second as well. Also, you can have a hard time knowing which event sheet is causing the problem. If you need to use the same events on multiple event sheets, you should put all the shared events in their own event sheet and include it in the others.
I suggest you redo the event system removing ALL the disable actions and instead use state to define which block of code to use.