Just to confirm it is the W10 app causing the problem, when it pops up again just click on it and it should open whatever app has the problem. If it is the W10 app and you are not using it, just delete the account from the app.
I've tried to go into the APP, think I've got it correct, it's the Icon that shows as an Envelope.
Gone into Settings/Manage A/c/BTInternet/
Went onto Password box to change this
Put in new password/clicked to save
But system just keeps cycling & does not save new password.
Tried again but this time can't even get into the BT account
Do you actually use the app as well as Thunderbird? If you don't, just delete the account from the app.