Check the warning log on the Oracle server, as well as free disk space at the destination of the archive log. This error can be caused by an archiver error when it cannot archive redo logs.
You can log in as dba and do: show parameter archive , as Justin explained in his comment.
You are looking for entries like log_archive_dest_N . You will likely have many of these options, and I would suggest checking every drive / location that matters.
My database has log_archive_dest_1 = '/o296/dbSID/arch'
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