Some questions that spring to mind:
- What is your firmware version?
- How are you trying to sideload the books? Drag and drop? calibre? ???
- What is the source for these books. Quite a few "free" sites are not noted for the quality of their content. Or legality but that's a discussion for another day.
- Have you error checked the books? Four suggestions are using calibre's editor with it's epubcheck plugin, Sigil with it's epubcheck plugin, using one of the online epubcheck sites or—if very enthused—you can download the epubcheck software and run it on your PC. 1, 2 and 4 will require having Java installed on your local machine.
- Have you tried loading a few books at a time to see if you can locate the book or books that are triggering the issue? Kobo's firmware is very unforgiving of Garbage In.