Fixed the above myself by putting the forums on a mandatory prefix [OPEN] and added that prefix to the "solved" prefix set.
But there is another problem/bug with this mod.
When setting a thread to solved, the prefix will be set to solved, but the forum information is not rebuild immediately. This way the "Solved" prefix is only visible in showthread and forumdisplay but not on forumhome!
I explained here:
https://vborg.vbsupport.ru/showthread.php?p=2110759
How can I fix this?