External SD card not recognized, works fine in recovery

[Android Central Question]

I have an external SD card that used to run fine in my phone. All of a sudden, it was no longer recognized by the system. In Settings -> Storage it is not listed at all. If I open a shell (via adb) I can, however, see that there is a device /dev/block/mmcblk1 with a partition /dev/block/mmcblk1p1. That is my SD card! I can even mount it to e.g. a new directory /mnt/ext_sd with 'mount /dev/block/mmcblk1p1 /mnt/ext_sd' and then access the files. No application apart from adb seems to see the files in /mnt/ext_sd, though. If I reboot to the recovery, the external SD card gets mounted normally and I can access the files. How can I fix this? I already tried rebooting and cleaning the SD card.

BQ Aquaris X, Android 7.1.1 (Nov. patch), Stock ROM, Rooted (Magisk), TWRP