Network user can't edit/save files in workgroup folder

Suddenly I can't edit/save files in my workgroup folder on OS X Server (but I can copy, delete, duplicate files) ...

I use a Mac Mini Server (2011) running OS X Server 10.8.2 and OpenDirectory with network users defined. File operations were fine for the network users (creating, copying, deleting, editing files in a joint workgroup folder on the server) from my MacBook Pro (10.8.2) but suddenly I can't edit/save files any longer to the workgroup shared folder. This changed from one document to another, without logout or reboot. I also tried different applications (Preview and PDFPen) or a new workgroup folder with the same result.

The error message I get in a pop-up:

"The document xxx.yyy could not be saved. You don't have permission."

However, I can still copy, duplicate and delete files from the same MacBook Pro.

The log file says :

03/02/2013 13:34:16.212 NetAuthSysAgent[588]: NAHSelectionAcquireCredential The operation couldn’t be completed. (com.apple.NetworkAuthenticationHelper error -1765328228 - acquire_kerberos failed testuser8@LOCAL: -1765328228 - unable to reach any KDC in realm LOCAL)

When I try to do the same from a Mac Pro (10.7.5) I have no problem. I have tried it with several accounts including a newly defined user. Results on both clients are still the same.

The only message I did discover related to authentication is the following (but it also appeared before things went wrong):

03/02/2013 08:26:33.905 kdc[48]: WARNING Found KDC certificate (O=System Identity,CN=com.apple.kerberos.kdc)is missing the PK-INIT KDC EKU, this is bad for interoperability.

What can I do. I considered restoring my MacBook Pro from a Time Machine backup but as long as I don't know whether it is server or client related I'm hesitant to do so.

Thanks for the help,

Erik