I awoke this morning to find the same '500 - Internal Server Error' after it appeared to update overnight.
In checking further I found a stock /etc/apache2/mods-available/jk.conf had overwritten the UT file, resulting in mod_jk not working.
Replacing the file with the jk.conf.dpkg-old restored access to the WebUI where I could see there was still an upgrade available. After completing the upgrade I see it also replaced the jk.conf properly.
Not sure what happened or why there was seemingly a partial upgrade resulting in the UI becoming unavailable but all good now - just in case someone has similar.
C.