I had the same problem in two different sites. Also using weaver xtreme in both, and I tried all the things mentioned here before (deactivate, remove, reinstall plugins, create new menus, firewall, hosting, etc) but nothing worked. Finally I solved (not found the root problem, but certanly I think it’s theme related) deactivating, NOT REMOVING, my weaver theme, activate Twenty Seventeen theme, I create/modify my custom menu, and the changes were finally saved. The 403, 404 errors gone immediatly. Then, when the menu was in the way I wanted, I just activate Weaver again and change the primary menu to the one I need and Voila! Everything is working fine. By the way, once the menu was saved under Seventeen theme process and switching to Weaver, I was able to modify/add/create new items and menus under Weaver theme without any problem.
Hope this can be helpful for the people who is suffering the same issue (I found about a dozen similar questions trying to fix this)
