the problem is/was that the way to tell SynthHihol to syntax-highlight a given part of the post. you have to put that part between <pre lang="language-name"> and </pre> this is quite reasonable, but the problem is that markdown’s <pre> is not customizable, so there’s no way to add a language-attribute to it.

later, when i was searching for an alternative syntax-highlighting plugin for wordpress, i found wp-syntax. unfortunately it had the same problem. but then i asked about my problem on the wordpress-forums, and got an answer .and it works.

while the answer is for wp-syntax, probably the same thing would work also for SyntHihol.

i also switched from SyntHihol to wp-syntax, because it seems to be better maintained.