Warning: There are too many active tabs (11)
 

What exactly does this mean? How do you fix?

Screenshot?

In the navigation manager check and see what the last active tab listed in navigation manager, edit that and save (make no changes just click edit then save it). Clear the system cache... resolved?

Edit: It could be the mod doing the second navbar i.e. the db tech mod but not sure so I'd check the mods thread for anyone else who posted this/similar error.

That shows up in the navigation manager if you have more than 10 active tabs. But I did a test with 11 tabs, saw that message displayed, but they all seemed to work. So that's a good question. I think Paul M. should know.

All that means is it could cause display issues with that many tabs showing in the navbar.

Since you are using the DBTech mod, and have multiple navbars, it is safe to ignore it.