We seeing a major issue on different sites.

Problem is,, we can't really make out what/why it happens;

Updating to 4.91 Sp4 works Ok on the majority off system's but;

1. At one client's site; approx 10-20% of the machines refuses to
login when Sp4 is applied. After the loginbox has been filled out and
"clicked" a black screen appears and ....nothing more..., just sits

Workstation only works on those systems.
Back-rev to Sp3 also works on those systems
( and off course,, worked before beeing upgraded to sp4)

We found the cause AND solution;
Windows Group Policys.
If they are there,, regardless of what's in them, the above scenario

If we do workstation only, remove group policys and then restart,
login works again.

BUT;,, if we edit the grouppolicy's in ANY way, regardless what,, it
stops working again.

We also notice that the problem seems to occur only on the newest
systems ( Intel OEM'd desktops all of them). All systems beeing 1
generation older does NOT have this issue.

Site 2;
actually,, my own PC.
I've been running SP3 without issues and then also a couple of beta
versions of SP4 with no issues.

Today applied the SP4 release version and got a black screen instead
of any login box, it never appears.

Solution for me was the same as in the above case,, simply remove
group policy's. In my case,, Group policy's had only a setting for
clearing the pagefile upon reboot, nothing else.

I couldnt even get a loginprompt,, so it was safe mode / remove client
sp4 completly, then restart remove GP files, reinstall SP4 and it

Bottom line; very new PC's with existing group policys can't login if
updated to SP4.

Why ??,, no idea whatsoever...