Still do not know why -
but it works now. I reinstalled it and purge and so on (this presumably was the reason that install was 'not compatible' - I still have had the same (?) version installed),
after this I installed the here downloaded libwebp.deb with dpkg -i in superuser-mode (after it told me that this is neccessary) and installed the ppt (for verification it works) through the filemanager.

Now Telegram starts fine again.

Still wondering what the reason was...missed I to 'purge dpkg' the first time or is there any difference between the files here and the repos-files?