The category cover only available for the top level forums, for the blue panels in wpForo > Forums admin page. There is no way to set cover image to forums and subforums, only categories can have cover image.
4VM1501 admin.php:505 Uncaught TypeError: wp.media is not a function
at HTMLAnchorElement.<anonymous> (VM1501 admin.php:505:74)
at HTMLBodyElement.dispatch (load-scripts.php?c=1&load%5Bchunk_0%5D=jquery-core,jquery-migrate,utils&ver=6.0.1:2:43064)
at v.handle (load-scripts.php?c=1&load%5Bchunk_0%5D=jquery-core,jquery-migrate,utils&ver=6.0.1:2:41048)
DevTools failed to load source map: Could not load content for chrome-extension://eakacpaijcpapndcfffdgphdiccmpknp/scripts/content_page.map: System error: net::ERR_BLOCKED_BY_CLIENT
4VM1501 admin.php
The problem comes from 4VM1501 admin.php file, the name says nothing but if you click that file or hover the mouse on the file name we could see the full path and where it comes from.
In any case, I didn't see any report about the deactivating plugins one by one and checking. Have you done it? It's the only way to find the conflict maker. There is no any solution from wpForo side, because the problem maker is not wpForo. Just do your best and play with activating/deactivating other plugins to find the conflict point.