I am suddenly getting the same FlexLM error today May 16, 2015.
Althought, I can checkout a NX7.5 in native from the same license server. Which means the user can reach the license server. Same test in three different computers.
The FlexLM error issue only happens when trying to Login Teamcenter 8.3
What is the issue?
Check the UGS_License_Server variable in tc_profilevars.bat file,if the teamcenter you are using is 4T then check the network from application server