After setting up your CB GroupJive menu items create one to the All Groups page or All Categories page. That should remedy the 404 issue for the time being. I've to figure out the best way to go about this as the issue is with the Itemid matching behavior. Currently our Itemid usage functions as follows.

First it checks if the current page Itemid is a match for the currently rendered URL. If it's not it then checks the database to see if a menu item exists, but it does this per URL so is bad on performance. To combat this it does fallback behavior where it mainly checks for its root. So for example it checks for a menu item existing for the plugin and will use the first one found. This can be bypassed to only look for an exact match, but if you do that it won't fallback if there isn't an exact match.

It's a bit of a nightmare to explain, but the bottom line is I need to completely rewrite our Itemid function. It was bandaided awhile back to improve the performance some, but since the introduction of plugin routers it's just not ideal anymore. It's too easy to end up with URLs having either no Itemid or the wrong Itemid with our current implementation.

Have added a feature ticket for this improvement in CB. Once improved it more or less should just automatically fix any wrong Itemid usages since it's all handled at URL render time.

forge.joomlapolis.com/issues/8010

I am not getting 404 errors from CB GroupJive menu items. CB Activity is constructing URLs the same way the rest of GJ is. Please PM backend super administrator login credentials with steps to see your exact issue and will take a look.

The menu item it is falling back to has an Itemid of 6778. That menu item is the following.

Plugin: CB GroupJive
Action: All Groups

That is working exactly as it's supposed to. It will fallback to the last available "All Groups" menu item. Your menu item 6678 is also an All Groups menu item, but 6778 is the last available so it'll be used instead. None of this is producing 404s on your site and can access groups as well as their categories perfectly fine from CB Activity. At this point I still don't understand what the issue is, sorry.