This has to do with
CHARSET'] = "iso-8859-1
and CHARSET'] = "utf-8
In the language files and database that collation do not match for iso-8859-1.
You can search here for diamond for a start.
Like
https://www.ubbcentral.com/forums/ubbthreads.php/ubb/showflat/Number/257369
https://www.ubbcentral.com/forums/ubbthreads.php/ubb/showflat/Number/262227
https://www.ubbcentral.com/forums/ubbthreads.php/ubb/showflat/Number/259803

Then you will need to make a decision to either switch back to iso-8859-1
or attempt to convert to utf-8

Basically if you wish to stay at iso-8859-1
GOTO the UBB CP select languages and then edit generic.php
The first entry probably has utf-8 then just change it to iso-8859-1
save and exit.