UPDATE: Sorry, not a solution either.
I might have found the fix for this problem. This thread led me to the underlying cause:
I followed the documentation (https://download.seafile.com/published/seafile-manual/deploy/using_ldap.md) in order to enable TLS for the LDAP integration.
Therefore, I created a directory disabled_libs_use_local_ones_instead here:
${SEAFILE_INSTALLATION_DIR}/seafile-server-latest/seafile/lib
Apparently, I created the directory as root. I’m sure it was readable for the seafile user. However, after changing the owner to seafile, the intermittent errors seem to be gone for good:
chown seafile:seafile disabled_libs_use_local_ones_instead