Problem with Picture-paths with ~…

my blog-url is:
https://www.unet.univie.ac.at/~a0404420/php/index.php

Now I’ve got the problem, that I cant add Pictures with the regular blog-editor (which means I cant just drag&drop them to the editor after I uploaded them) because after saving it the Editor changes the code of the URL:
When I drag it into the windwo the correct code is:

https://www.unet.univie.ac.at/%7Ea0404420/php/wp-content/112_1201.thumbnail.jpg

%7E must be the code for ~, but it works fine.. The problem is, after saving it changes the url somehow to

https://www.unet.univie.ac.at%7ea0404420/%7Ea0404420/php/wp-content/112_1201.thumbnail.jpg

Which of course breaks the link. Where does that second %&ea0404420 come from?? Which part changes it, why, and how I can stop it.

Problem is, that not just experienced users, who can change-back the html-code directly, are going to use the blog, so I really should find a way to fix it!

thanks for answering!