Good Morning Dominik,
Thank you for your question!
I have seen this type of error before, and you are given the option to export any errors to a spreadsheet. This is something I recommend - I recently saw a similar issue where a customer was using Google Enhanced campaigns and we found hidden characters once we reviewed the actual Display URL as it came over. A simple conversion to plain text revealed the hidden characters and we were able to resolve the issue. The same issue may also sometimes occur if the language is not a supported language. May I recommend this for the URLs you are trying to bring over - an alternative is to use the find a replace feature as well? This way you know that you are entering plain text as it comes over. Please do not hesitate to contact us at support so that we can look at your import directly.