Start a new thread, but [5.0 beta] in the thread title & follow https://www.zotero.org/support/reporting_problems including more details & an error report ID. Note that BBT is currently not working in 5.0, so that'd be a possible cause.
Translate: Beginning translation with Better BibTeX
That's still importing with the BBT translator. You have to remove that separately. Easiest way is by resetting translators from the Advanced → Files and Folders pane of the Zotero preferences.
If there's a way to have this action take place automatically when BBT is uninstalled or disabled, that might prevent this error from being reported again. I don't think I would have found this reset button otherwise, which does not itself mention BBT.
Note that we don't have anything to do with BBT — you would have to suggest that to the developer. (But I agree that it should do that automatically if possible. I actually thought it did that for uninstalling, but not sure, and not sure about disabling (or if that's even possible).)
I understand your point and I will follow up with the BBT team. However I would still like to suggest that since the plugin system is part of Zotero, ideally when I click uninstall that is what should happen. To put it another way it should not be possible for system preferences to reference features of plugins that are not installed (or have been disabled - otherwise they are not disabled!). Anyway I think we are on the same page and my problem is solved now, so thanks again :)
Upgrade Storage
I currently cannot import .bib files
Is importing either .bib or from clipboard a BBT feature exclusively? If that's the case then that would appear to be my problem.
Is there any other way to import?
Can you provide a Debug ID (different from a Report ID) for an import attempt that fails?
I should note that I have uninstalled BBT for the time being.
If there's a way to have this action take place automatically when BBT is uninstalled or disabled, that might prevent this error from being reported again. I don't think I would have found this reset button otherwise, which does not itself mention BBT.