Buying an OS map tile has stopped my OS maps working altogether

I've just bought an new Ordnance Survey map tile (Peak District, White Peak, if that makes any difference). When I try to display it, the map does not display, but instead I see the words "Unknown problem".

Worse, this seems to have killed all my existing OS map tiles. I cannot view any of my existing ones any more, although they don't show "Unknown error", they are just completely blank.

I have tried rebooting my phone, which didn't make any difference, and don't know what else to try.

I'm using Locus Map 4.29.0 on Android 11.

Please help!