Did you take a look at the following forum post: https://forum.mendix.com/link/questions/92516?
Hope that helps you fix your issue.
I desinstalled my SVN version and took the version 1.7.15 as it is advised in the question : https://forum.mendix.com/link/questions/92516 .
I now got the following error when trying to commit or update from my app :
SharpSvn.SvnWorkingCopyException: This client is too old to work with the working copy at 'C:\Users\drayd\Documents\Mendix\Customer_Data_Form V2-main\theme\styles\css\custom' (format 31). You need to get a newer Subversion client. For more details, see http://subversion.apache.org/faq.html#working-copy-format-change
You do not need to versioncontrol css files, since they get overwritten during every deploy by the scss-files. Don’t know if it will work at this status, but you can try to right-click the css-directory and us the option ‘Unversion and add to ignorelist’