Hello Franz,
I don't have a "clear" answer to your question; all that I can offer is a willingness to help the work towards an answer.
My current big obstacle in this direction is the seeming contradiction between "A reboot of the client does sometimes solve the problem, but often not." and "What helps is to restart the IP-Helper Service".
Surely a reboot also (re-)starts the IP-Helper Service. Is there really a difference between these two ways of "restarting" the service?
The reason that I ask is that "tracing" (with Event Tracing for Windows (ETW)) is how I often solve problems. From my limited understanding, it would be helpful to believe that any restart of the IP Helper Service was potentially/equally useful (tracing a service restart is easier).
A quick binary inspection of iphlpsvc.dll suggests that error 0x643 is only generated by the routine iphlpsvc!IpTlsAddInterface - it would be reassuring to believe that tracing the behaviour of this routine (regardless of whether via reboot or service restart) was equally insightful.
BTW, you probably meant to title this thread "Intermittent" (zeitweilige) Direct Access connection failures rather than "intermediate" (zwischenliegende) failures; it did not stop me from reading the message (mention of "Direct Access" was enough), but it might confuse some readers.
Gary