Hi Thilina,
Do you mean after you change the user’s password to a complex password and sync again, the issue still exists?
Can you use this account and password login to the portal?
You can also try the following steps to narrow down the root cause of the issue:
1. Assign the Full Access permission to other mailbox.
2. Login to the mailbox and use Open another mailbox feature to see if it can display.
Please also check the license of this account.
Thanks,
Jason Jiang