There is a known issue in OMSA with IE 11 after IE update of October 2019. This is related to JAVA libraries used. This issue is fixed in latest OM builds. As there is no OMSA release for Windows 2008 R2 post 8.5 we have below workaround. 
I just accessed OMSA on a Server 2008 R2 system, PowerEdge R730, and can't log in; it just keeps bringing up the same login screen over and over, no errors or other messages.  I tried upgrading from 8.1 to 8.5, but it didn't help. System is running 11.0.9600.19596 (Jan 2020 patch).  I found another R730 on the same network but different domain, which is able to access the first's OMSA without issue; it's running IE 11.0.9600.19431 (Aug 2019 patch). Is anyone aware of any changes in IE between Aug 2019 and Jan 2020 that might have caused this issue, or some setting default that's been changed?  I compared the IE settings and GPOs, and couldn't find any notable differences. Thanks in advance!   P.S.  I opened a similar ticket a couple days ago for Server 2016, but there upgrading to OMSA 9.4 fixed the issue; 8.5 is the highest version for Server 2008 R2.
OMSA 8.x - Login Loop