Error 8506 at client side

Since updated to groupwise 8sp1 (server) we have problems with two
notebooks (lenovo t500) that have been uptated to GW8 client.

Users works most time via proxy in central mailboxes and there they have
to start special formativ macros to archive those mails to a special
system (macro will store the mail with control files into file system on
server and delete them from mailbox). Using this function they get most
often 8506 errors.

Users that still use 703 client have no errors.

any ideas?
Axel