Let's see if some game files are corrupted...
Could you copy osume.exe to an empty folder in your PC? Any empty folder is fine.
Run the file: it should redownload the entire game in this folder.
When it is finished, download a .osz file (try with IE or another browser this time, you never know) and put it in the new Songs folder.
Run the new osu!.exe and tell us if it imported the beatmap without errors.
(you might disable your antivirus during the entire process for maximum chance of success)
Could you copy osume.exe to an empty folder in your PC? Any empty folder is fine.
Run the file: it should redownload the entire game in this folder.
When it is finished, download a .osz file (try with IE or another browser this time, you never know) and put it in the new Songs folder.
Run the new osu!.exe and tell us if it imported the beatmap without errors.
(you might disable your antivirus during the entire process for maximum chance of success)