It's working! Can't believe it ^^ It was in fact just a problem with the fuses as discussed in the link I posted.Here's the complete fix step b step for future reference since I as a beginner had some trouble following it correctly:1. Copy the code from this post2. Go to C:\Program Files (x86)\Arduino\hardware\arduino\avr (or whereever Arduino is installed on your computer)3. Open the boards.txt in that folder and paste the code in it, save the file (you might have to remove write protection)4. Restart Arduino IDE5. Now there should be an additional processor available under tools/processor called "ATmega2560 (Testato ISP Version", select that processor6. Burn the bootloader once with these settings7. That's it! Now uploading sketches with USBasp works flawlessly Thanks go out to Testato and NicoHood for figuring out this fix!