Need help: Local Server does not work continiously

for months I had no problems with my Rasberry Pi3 in connection with several NodeMCUs collecting data and show it on iPad or iPhone.
Since several days there is no continious operation of the Rasberry. I have missing points in the diagrams.
Login in my Account on the Rasberry is sometimes very fast (in this case Data is transmitted from the devices via Server to my iPad, sometimes serveral logins are nessecary, sometimes login doesnˋt work at all.
After a successful login it takes minutes to hours until the connection stops again.

Server is connected by WLan to my router - LAN Connection shows the same problem.
Activities: OS from Rasberry resetted, latest Blynk server software, latest Blynk App, all devices with latest Software.
Any advice or ideas?
Thank you in advance.