Check the files in your game folder manually, chances are something in there isn't supported by RMXP. Just a theory though. Also, have you ever self importated a
file into rmxp like lets say you went through this.
File won't go into rmxp, but you really want this character file. So you take the file, copy it then go into your game file in rmxp (or wherever you keep it) and
use ctrl + v (or paste with right click) and pasted in there.
From what I understand following that method I just typed up, it could cause errors within RMXP. Is that something you did, otherwise I would check
what you importaed into RMXP. It's been awhile since I've worked with RMXP, and I haven't worked with RMXP in forever now, so this is just a theory.
Also P.S.
Hopefully a mod will move the folder but this actually goes in the RMXP Troubleshoot folder, or something like that. Its in RMXP folder
or so on the main forum page (It's been awhile, since then and things have changed so I cannot be sure what's it called) but I'm sure a mod
will move this.