@Improv-Jester
You could try reading it with some XML validating tool to check for syntax.
However, I would suspect that the XML syntax is legal but it contains something the uic does not like. If you are really lucky you could try running pyside6-uic on it yourself in a terminal, leave output to stdout instead of file, and be able to see the code generated as it reads the file, so you would know how far it gets. However, it's quite possible that does not produce any output until the whole file has been read and parsed, in which case it won't help.
Other than that, the only obvious way is to start "binary chopping" the .ui file contents. That means, you remove about half of it, obviously in a suitable place leaving valid XML. If that run through OK you repeat process; if it errors you restore the removed stuff and chop that again. Till you find the problem.