"Exception has been thrown by the target of an invocation. "
After reboot their machine, they are able to login to QC project again. However this issue occurs frequently.
Here is the Windows setting that causes the issue:
From PC's Desktop go to <Start><Run> and Type 'secpol.msc'<OK>.
(This will get you the 'Local Security Settings' Dialog Window.)
Opt <Local Policies><Security Options> and scroll to the bottom of the list.
The Fourth Policy from the bottom of the list, 'System cryptography' must be set to 'Disabled' to allow a Quality Center proper Log on.
If they implement FDCC policy , which has the 'System Cryptography to use FIPS policy' Enabled, they will not be able to use Quality Center .
NOTE: This Policy affects the .NET tool to work properly with the Quality Center App.
Request a 'Deviation' for this Policy under FDCC and set 'System cryptography' to 'Disabled'.
After that Quality Center has no problems with the relaxed version of the FDCC Policy.