Lots of complaints about this
Can you use https://accounts.o2.co.uk/signin?
If so delete the App and reboot the phone before reinstalling.
Any further help
You will need to contact O2.
Message them on Social Media. Facebook (https://o2uk.co/O2CFB) , X Twitter (https://o2uk.co/O2CTW) or Instagram (https://o2uk.co/O2CIG)
Or phone them on 202 or 0344 809 0202 (contract)
4445 or 0344 8090222 (PAYG)
This is not O2 and we are all customers here similar to yourself and cannot answer account type queries.
