If you try to print labels and received an "UNEXPECTED_EXCEPTION" error, it will often be due to high load on the carriers API.
Another reason may be that one of the orders you tried to print a label for doesn't have a Package assigned.
Have a look through your orders, find any with a missing Package, assign one and try printing labels again.
A missing Package can occasionally occur when there is a system delay in adding Packages on order import, or a connectivity issue.
If the order returning the message already has a package assigned, wait a few minutes and try again.