That's because you install ocs in same computer before and not removed clearly, therefore the system may be getting confused and giving a Null value.
Please check by adsiedit.msc:
cn=LS AV MCU,cn=Microsoft,cn=servername,cn=computers,dc=..,dc=..
attribute: msRTCSIP-MCUData
You may see two value:
MCUURL=https://oldserverfqdn:444/Liveserver/AVMCU
MCUURL=https://serverfqdn:444/Liveserver/AVMCU
Remove the old one.
Thank Rajeev Krishnan of Microsoft.