OpenClient message: LAYER = (0) ORIGIN = (0) SEVERITY = (78) NUMBER = (35)

Hi,

I am new to freetds and have some questions about intermittent error
below that we get from our monitoring system that uses freetds for
SQLServer connect check:

ALERT                  : Unable to connect to instance
ERROR TEXT             : OpenClient message: LAYER = (0) ORIGIN = (0)
SEVERITY = (78) NUMBER = (35)
Server <server_name_here>, database Message String: SQL Server
connection timed out.

Every time when I get above alert
- I can successfully connect to SQLServer
- do not see any errors around that time in SQLServer log
- moreover even SQLServer default trace file does not show any
activity happening around that time for some affected servers
- issue affects different servers at different times (I do not see any pattern)
- other databases on the affected instance that are checked also by
monitoring system do not have same connection issue
- it is always one-off and intermittent issue (next run of monitoring
check scheduled every 15 mins does not have this connection issue)

Question are
1) What does "LAYER = (0) ORIGIN = (0) SEVERITY = (78) NUMBER = (35)"
error mean and what can be the cause of this error?
2) I am not sure if it is genuine connect issues (some intermittent
network problems) or freetds bug/wrong configuration/high load (we
check hundreds of databases each run)

Please advise.

-- 
Best regards, Eugene.