Radius Access-Request - No Reply from RADIUS Server after long conversation

Question

I have a HP Switch that acts as an authenticator for MAC-Auth. It sends Radius Access-Request to my Radius Server. THe Switch and the Servere are talking in Access-Request and Access-Challenge for a great while (8 packet back and forth) then the Radius stops answering the Access-Requests but also do not provide no Accept or Reject.

The Wireshark capture looks like this:

309 11.833685 Switch (NAS) Radius Server RADIUS 415 Access-Request id=153
316 11.839459 Radius Server Switch (NAS) RADIUS 162 Access-Challenge id=153
317 11.841418 Switch (NAS) Radius Server RADIUS 442 Access-Request id=154
318 11.843351 Radius Server Switch (NAS) RADIUS 132 Access-Challenge id=154
319 11.846635 Switch (NAS) Radius Server RADIUS 613 Access-Request id=155
320 11.848345 Radius Server Switch (NAS) IPv4 1514 Fragmented IP protocol (proto=UDP 17, off=0, ID=7481) [Reassembled in #321]
321 11.848345 Radius Server Switch (NAS) RADIUS 706 Access-Challenge id=155
322 11.850566 Switch (NAS) Radius Server RADIUS 442 Access-Request id=156
323 11.851065 Radius Server Switch (NAS) RADIUS 353 Access-Challenge id=156
325 12.015714 Switch (NAS) Radius Server RADIUS 612 Access-Request id=157
326 12.018087 Radius Server Switch (NAS) RADIUS 187 Access-Challenge id=157
327 12.023538 Switch (NAS) Radius Server RADIUS 442 Access-Request id=158
328 12.024162 Radius Server Switch (NAS) RADIUS 162 Access-Challenge id=158
29 12.026827 Switch (NAS) Radius Server RADIUS 484 Access-Request id=159
30 12.027339 Radius Server Switch (NAS) RADIUS 177 Access-Challenge id=159
331 12.029429 Switch (NAS) Radius Server RADIUS 484 Access-Request id=160
332 12.031112 Radius Server Switch (NAS) RADIUS 193 Access-Challenge id=160
333 12.034656 Switch (NAS) Radius Server RADIUS 538 Access-Request id=161
341 12.039336 Radius Server Switch (NAS) RADIUS 214 Access-Challenge id=161
342 12.041858 Switch (NAS) Radius Server RADIUS 473 Access-Request id=162

661 16.818609 Switch (NAS) Radius Server RADIUS 473 Access-Request id=162, Duplicate Request
1029 21.818073 Switch (NAS) Radius Server RADIUS 473 Access-Request id=162, Duplicate Request
1134 26.818118 Switch (NAS) Radius Server RADIUS 473 Access-Request id=162, Duplicate Request

On the NPS, following error message is visible:

Reason Code = 1, An internal error occurred. Check the system event log for additional information.

Even without any conditions (we made a policy where everything should match, like the only filter is date and time or the erxact IP of the NAS) and enve though we made sure that the packets match for the correct policies, the error message is always the same. 802.1x with Certifikate works

Any help is much appreciated.