OK, if someone can explain this I would be very grateful!
I've pretty much built my prototype library subscriptions app. Everything was working, I was super happy.
I came up with one more idea to increase functionality - have icons on my People gallery cards to indicate whether or not the person in question is a trainee.
Added a column to my datasource (just a yes / no response), added the card to my People detail form and People edit form. All fine, all working.
Went back to the gallery page and added the icon in the cards; my idea was to make it display only on people records where the trainee status is set to "Yes". I'd done something almost identical previously with a display icon to indicate whether or not someone was on leave / secondment, and that worked with zero problems.
The icon worked, BUT having now gone back to my People details form, I'm getting error messages and no data displaying. The message is:
Incompatible type. The "Trainee" column in the datasource you're updating expects a "Text" type and you're using a "Control" type.

I switched laptops the other day and didn't realise the Autosave feature had switched back on, so I can't undo what's happened (infuriating). My question is... what has happened?! Why would something that was working absolutely fine a second ago suddenly stop because I added an icon? The "on leave or secondment" icon and datacard caused no such issues, and I've literally repeated the exact same process - it's all practically identical:

I've gone and undone all the visibility settings for my icons so they're all showing and filled green for the time being (the triangles are going to represent whether or not someone's a trainee):

Little help? I'm about ready to hurl this machine out the window...