lineageos – file system error while writing

I've recently encountered a problem using Threema on LineageOS 15.1: I've noticed that every time I try to download an image posted by another person in a conversation, I get the message "Message could not be downloaded could be displayed. A logcat reveals that an exception is thrown when the file is opened:

java.io.FileNotFoundException: /storage/emulated/0/Android/data/ch.threema.app/files/data.blob/017c31a18e3325c75d3564fd3565b144

I used Adb Shell (as root) to create an empty file:

# Who am I
root
# echo ""> /storage/emulated/0/Android/data/ch.threema.app/files/data.blob/017c31a18e3325c75d3564fd3565b144
can not create sh: /storage/emulated/0/Android/data/ch.threema.app/files/data.blob/017c31a18e3325c75d3564fd3565b144 Invalid argument

The same error occurs in the shell, even though I am logged in as root.
After the file has been created, it seems that it can no longer be written:

# echo "asd"> 017c31a18e3325c75d3564fd3565b
# echo "asd"> 017c31a18e3325c75d3564fd3565b
sh: 017c31a18e3325c75d3564fd3565b can not be created: Invalid argument

Permissions on newly created files are set to

-rw-rw ---- 1 u0_a71 sdcard_rw 4 2018-12-31 13:45 017c31a18e3325c75d3564fd3565b

Any ideas, what causes that? Is this a problem with the misconfiguration / authorization? Or is this a problem with the file system (I have used both f2fs and fat32)?