@ray: Thanks for the recommendations!
I upgraded to 2.1.9 and then did a factory reset (without configuring anything else). I noticed again network delays and connection issues.
After approx. 10 hours, I imported the production configuration. The network problems continued. Actually, I experienced total (permanent) loss of connection and therefore I had to restart (power-off and on) the controller. Therefore, I returned back to 2.1.7.
It seems that there may be an issue with the Ethernet library. Could be a compatibility issue of the library Ethernet implementations with the router’s one(?)
Awaiting for any further news/suggestions.