Update: after rolling out the above workaround to a number of workstations, the problem is alleviated but unfortunately not resolved.

• On some workstations changing the setting has no effect. The problem continues to occur whenever the workstation is locked.
• On all other workstations the workaround fixes the problem only when unlocking the workstation after it has been locked manually ([Windows]+[L]). Th problem still occurs after the workstation was locked automatically by the screensaver.

P.S.: Disabling or removing Avast does also not fix the Problem.

Update2: The fix is not needed on an otherwise identical workstation that does not have AutoAdminLogin enabled. Unlocking after both manual and automatic locking of the workstation works as expected with default client settings.

I believe the key may be the "eDirectory prompt, Autologon to Workstation" you mentioned.  Meaning in addition to the Microsoft AutoAdminLogon policy being configured, the Client for Open Enterprise Server's option for "Prompt for network logon during Windows AutoAdminLogon" is also enabled.  (Also known as the Client for Open Enterprise Server "AutoAdminQueryNDS" registry policy.)

We are currently investigating a hang that occurs in the current client releases when both AutoAdminLogon and AutoAdminQueryNDS are both enabled. There is no known workaround for this issue at this time, other than to temporarily disable at least the "AutoAdminQueryNDS" policy which seemed to avoid the failure in our investigation thus far.  But the actual root cause has not yet been identified yet, nor is there a proposed fix at this time.

Has there been any progress on the issue? The problem persists on two of the affected workstations, even after moving to client version 24.1.

It is something which is actively under investigation right now, and the design for the fix has been identified.  It was hoped that this fix might make it into the next available release such as 24.2, but the design as proven involved enough that it likely will not be completed and tested within a time frame suitable for 24.2.  But the work is actively occurring, and should be addressed as soon as it can be.

Until then the only possible workaround in current shipping clients is still the disabling of the AutoAdminQueryNDS and Windows AutoAdminLogon policy.  The issue appears to have started with a change made in the Client for Open Enterprise Server 2 SP6 time frame, so it would require an SP5 IR2 or earlier client to attempt working around it in a manner which allows the AutoAdminQueryNDS policy to remain enabled.