I get Access Violation after opening either Diamondus Ultimate or Jungle Ultimate...

Hello! I'm going to guess that you're trying to open them in Jazz Jackrabbit 2, because I don't associate the Access Violation error message with any level editor software. What version of Jazz Jackrabbit 2 are you using? You can see the version in the bottom left corner of the root menu, the one that shows up after the title screen. If it doesn't say "JJ2+ Beta v5.12" you will need to install JJ2+ 5.12 before Diamondus Ultimate or Jungle Ultimate will work. You can read installation instructions here.

Unfortunately it does say "JJ2+Beta v5.12 (2023-08)"...

The strangest thing of all is this. First I only had Jungle Ultimate and it ran properly. But like 3 days ago I downloaded Diamondus Ultimate that did NOT run (Access Violation)... and from then on Jungle Ultimate didn't run either...

Ah, okay, so this is something more specific. My next guess is that you're running into the issue that Diamondus Ultimate and Jungle Ultimate have several files with the same filenames but different versions, particularly DD-Anims.j2a. Try extracting all the files from one of the zips, overwriting any files already in your JJ2 folder that have the same filenames... that should force your game to use the versions of the files that each level expects.

Because Jungle Ultimate was released later, I hope that it accounts for this issue, and hope that installing Jungle Ultimate after Diamondus Ultimate (overwriting everything) makes both of them work at once, but I didn't make these files, so I don't know for sure.

That did the trick, thanks man!