First: your fix didn’t help. Sorry. Or it did — that error was a little bit strange,
Please, try to follow me 
I have 5 channels:
- three for TL1
- one for staff
- one for custom group of 5 persons
I moved all others than staff-channel from sub-categories to top.
Then I tried how category preview component works, but it wasn’t what I needed, so I disabled it. Right after that I did rebuilding to get category lockdown plugin.
It worked, except I had some weird issues with FKB Pro theme and login/signup-modals — those gave only spinning loading icon. That happened only with my iPad AFAIK.
At that point I realize the chat channel of custom group didn’t work any more. Plus staff-group and 2 of 3 TL1-groups showed only very recent messages. One TL1-group was intact.
I tried safe-mode and disabling most of plugins via settings. No help and I opened this topic.
Long story short and after several rebuildings removing and re-enabling plugins:
- staff-channel and two TL1-channels are low traffic ones and I forgot totally I’m using 14 days rule there; so there wasn’t any issues
- custom-group-channel was broken if I had both plugins Data Explorer and Category Lockdown installed, and enabling/disabling didn’t matter
- rebuilding without those two plugins fixed that one broken channel. When I re-rebuilding (is that a word?) with both plugins that channel was still ok
My un-educated guess is something went sideways when I first tried Category Previews, then I disabled it, then rebuilding with Category Lockdown and the last move was deleting Category Previews. Fixing some bizarre SQL-errors or false bites needed totally removal of Data Explorer and Lockdown, following fresher re-installing.
What I can’t understand is error log of Discourse — how do it fit in all of this.
Anyway. I have all plugins back online and channels work nicely.