It happens to me as well and quite a lot recently.
In order to resolve, i usually reset my password and then i log back in. Funnily, i always reset to the same password (ie: the one that is meant not to be valid anymore) and it works.

One thing i found out though: it only happens when i'm outside of the UK. I'm wondering if maybe the BA app thinks that the non UK network i am on (when it happens) is showing security conflicts that sort of lock the account on the app?
Does it make sense?