Looks like the fix has solved the main issue but there is still a minor incompatibility problem. nh frequently tries to establish additional bridges and occasionally fails to get it over the "min. working difficulty" of 65535. We still have to pay for the failed connections. According to the Order Details page the Delta value is between -1 % and -3 % all the time (Delta: "difference between measured pool speed and paying speed.")

Meanwhile I had opened a ticket at nh. The response was as follows:
... Minimal difficulty is enough so the order is alive, but as soon as it gets some miners, pool needs to reach working difficulty or above. If not, the order will be disconnected. We suggest to simply use pools that never use difficulty below 65536 and you will not have issues with that.

Furthermore, the Pool Verificator at https://old.nicehash.com/poolver.jsp?a=1&l=0
comes up with: