Thanks!
What i did: i first formatted the SD card with SD formatter. Then used diskpart to clean it intirely, using this tutorial
https://www.[Suspicious link removed]s/www.windowscentral.com/how-clean-and-format-storage-drive-using-diskpart-windows-10%3famp
After that i could write the X10 file to the SD card succesfully. Put it in one of the baikals.....and tadaaaaa!! It's working again!

Thanks for your help. Tomorrow i will do the same with the rest of the baikals and give an update when they are all on air.