Everything was fine as long as I used CyanogenMod 11 (Android 4.4) on my HTC One S. I recently switched to LineageOS 14.1 (Android 7.1.2) and since then couldn't use its non-removable 16-GB sdcard as an internal storage.
Now there is
/storage/emulated/0 instead of it. I logged in via ADB and mounted the card inside this directory (
mount -t vfat -o rw,uid=0,gid=1015 /dev/block/mmcblk0p36 /storage/emulated/0/sdcard) and could browse it there, but still not on my phone. Permissions are similar for the mounted directory and the rest of the storage.