resuming server replication on PostgreSQL servers
It seems like archiving managed to partly write the WAL archive file before the space ran out.

Then the test in your archive_command will notice that there is already a fike of that name and will fail.

In this case the solution would be to manually remove that partially archived WAL segment so that the next attempt to archive it can succeed.

You might want to improve your archive_command by removing the file if cp fails (while still returning a non-zero return code).