Failed to communicate with "/interface-data/init?1584701349995", timed out after: 60

[triggerfish]

Since I reinstalled my raspberry pi and upgraded everything to most recent versions, I get the error trying to access my calibre server:

Failed to communicate with "/interface-data/init?1584701349995", timed out after: 60 seconds

I am asked for a user name and password, which is a request from the calibre server. After entering correc info (I checked using invalid data) it takes a while before I get the error. The server acces log says:

192.168.51.100 port-55841 - 20/Mar/2020:11:49:20 +0100 "GET / HTTP/1.1" 304 135
192.168.51.100 port-55841 - 20/Mar/2020:11:49:20 +0100 "GET /ajax-setup?1584701349861 HTTP/1.1" 200 370
192.168.51.100 port-55841 - 20/Mar/2020:11:49:21 +0100 "GET /interface-data/init?1584701349995 HTTP/1.1" 401 292

I run calibre server on a raspberry with raspbian 10.3 and run calibre-server 3.39.1. The library is on a mounted QNAP NAS disk. The nobrl option is set. This setup worked fine with the old raspberry install, but that was rather out-of-date and I decided to just reinstall everything, which I start to regret. There are no known blocking factors between my laptop and the raspberry, so... Please help...

Best regards, Peter