Sooo, I found this article:
https://wiki.mobileread.com/wiki/Repartition_the_Kindle
Couldn't hold my temptation and a quick
Code:
[root@kindle root]# mkfs.vfat /dev/mmcblk0p4
mkfs.vfat 2.11 (12 Mar 2005)
[root@kindle root]#
fixed it indeed!
So I'll get KUAL und usbnetwork to work again and then make backups of all the partitions first thing
Edit: In case someone has to do the same: Reset the Kindle from its device menu after formating, it fixed mounting issues for me