How to solve the problems to convert musicfiles in pdf-format to mscz ?

Hi !
I have for a long time used the functions to convert pdf files to mscz format from the URL

https://musescore.com/user/login?destination=%2Fimport

It has mostly gone well, except that I have received the message "unsuccessful" when the original PDF file has not been good enough.

But now I suddenly have problems with all pdf files that I try to convert being queued, but I do not get the message "unsuccessful". When I then delete the files that have been queued from Explorer and then restart the computer, I can go in and delete the files from https://musescore.com/import. If I then try to convert a new file, I get the same problem as before. What can I do to solve this problem ?