Hello there,
I posted this issue to the official support thread, too but no one replied. So I'm going to try a seperate Post.
I converted my YaBB 2.4 to an older version of SMF. I first tried 1.1.21 and then 1.1.14. I can't install an even older version because of my MySQL version.
The conversion was running without any error message exept for the attachments (because I didn't install the attachment mod package). Everything seemed to be absolutely fine. All of the thousands of Topics and Postings were converted.
But: Lots of Postings got cut off at the first unusual characters (Ä, Ü oder Ö). My first try was with UTF-8 charset, even for the board and for the conversion. My second try was the using of ISO-8859-1 for the converter an still UTF-8 for the forum. The third try was not using UTF-8 for the forum.
Lastly I installed the oldest version of SMF that I can install on my server. It's the 1.1.14. The results were the same like with 1.1.21.
Some of the Userpostings are complete, some are cut off.
Also the most of the User roles are missing (what is not the biggest deal at all).
What I don't understand is that some postings are converted completely and some are not. There are no diffrences between those postings on the YaBB 2.4