SUMMARY

Out of the blue, without touching anyting (although it might have been after some upgrade of some KDE component, not KMail), I started to get this message when trying to use the 'U' shortcut to mark messages as unread:
The key sequence 'U' is ambiguous. Use 'Configure Shortcuts'
from the 'Settings' menu to solve the ambiguity.
No action will be triggered.

I looked in the shortcuts configuration and there is nothing else bound to `U`, not even in the global KDE shortcuts defined in the KDE Settings.

STEPS TO REPRODUCE
1. Open KMail
2. Select an e-mail
3. Press the "U" letter in the keyboard

OBSERVED RESULT
A dialog with this message is shown:
The key sequence 'U' is ambiguous. Use 'Configure Shortcuts'
from the 'Settings' menu to solve the ambiguity.
No action will be triggered.

EXPECTED RESULT
The message is marked as unread.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Debian unstable, kde version 5:104 (from the debian package)
KDE Plasma Version: 5.14.5.1 (?). Is the version of the plasma-desktop package
KDE Frameworks Version: ?
Qt Version: ?

ADDITIONAL INFORMATION

If I remove the binding for 'U' from the shortcuts config and then press 'U', there is no visible effect.