@gertjan Pfsense uses 127.0.0.1 as it's nameserver (it was displayed then using the pfSense dns lookup tool). I checked all settings on my win10 client and even captured the packets with wireshark: The packets were definitly sent to pfsense and were processed there (i saw the specific lookup request I made in the unbound logs). Good idea to check the resolution with the cli, thx.
However in the meantime, it seems like it's working:
I have noticed that I didn't upgrade my pfSense for more than 3 months. Therefore I checked for updates and saw that the version 2.6.0 was available. I installed it and as of know, the problems are gone.
Don't know if this was a bug in the previous version or what, but it was definitly strange...