I have had the same problem since the beginning of August. I tried the recommended solutions such as switching off and on, removing and adding the btinternet account and eventually, but probably unnecessarily, did a factory reset which created a lot of work. The latter two solutions each worked for a couple of days then stopped.
Finally, I found the update to gmail dated 5 August 2023, downloaded and installed it.
It seems to have worked! This may well be a patch to solve the problem created by the previous update.
