That the problem is caused by the new kernel. If I replace the new kernel (4.9.58) by the old one (4.4.50), but still use the latest emteria.OS, the app is working fine.
Unfortunately, I see no difference in the kernel configuration between those two kernels. Maybe the ip filter rules have to be configured in a different way... More debugging is needed. I'll keep you up-to-date 😉