Hi,
Is your SQL cluster in DMZ? Is this backup session running longer than 1 hour?
If yes, then you should put these parameters in omnirc file (C:\ProgramData\OmniBack\omnirc) :
OB2SHMIPC=0
OB2IPCKEEPALIVE=1
OB2INETTIMEOUT=10
OB2IPCKEEPALIVETIME=900
OB2IPCKEEPALIVEINTERVAL=60