I was able to fix this issue in my project, I was upgrading from 9.14.1 to 9.24 and had a few marketplace moduels that depended on slf4j, during the upgrade new versions of the marketplace modules will install any jars they need in vendorlib as opposed to the old behavior which used userlib, (see dependency management in mendix docs).
So by after deleting any old and leftover jars from userlib which were related to log4j and slf4j and may have been conflicting with the ones in vendorlib the issue was resolved for me. Please try this solution!