I have the same issue.
I have been able to delete the BitLocker encryption keys many times before but this time - it's getting stuck on that pop-up as shown on the screenshot shared by @Cu89
I'm not that much of a genius to say the following:
I think this is a glitch of some sort because it only happened after I 'deleted' the registered device on my account before deleting the BitLocker encryption keys. In other words - when I now try to delete the BitLocker encryption keys - it is probably searching for the device to which these keys were registered to - but since it does not have said registered device on the account - it gets stuck on the that pop-up message and does not delete.
The above is just a wild guess - but could be true... do you have any input to add about this @S.Sengupta ? How do we let Microsoft know about this issue?