Searches blocked

Core Machine

Intel NUC 7i7BNH, 32GB RAM, 256 GB SSD (100 GB in use), 1TB HD (HD not in use)
Roon version 1.8 (build 790) (64 bit)

Network Details

Router Fritz!Box 7590
INTEL NUC (core)
Switch Cisco-SD2008v3
Meridian-861v4-ID40 (zone 1 with 2x DSP5200SE (see zone 2), 1x DSP3300,
1x DSW, 2x505with Braun L300 and 2x DSP33)
Meridian-218-1 (zone 2 with 2x DSP5200SE (MQA))
Meridian-218-2 (zone 3 with 2x DSP3200)
Apple-Mac-mini (zone 4 through Meridian HD621 (MCh))
QNAP-TS-451A with 4x 3TB, 3916 albums with 68191 tracks
Switch Zyxel-GS1900-24E
Meridian-MS200 (zone 5 with 1x DSP33)
and some more non-audio-related stuff
All afore mentioned audio stuff is cabled connected.
One SB Touch is connected through wifi (zone 6 with 2x AudioEngine2)

Audio Devices

Through wifi my controls are connected:
2 iPhones (S7 Plus)
2 iPads (2017)

Description of Issue

Already since more than a year my search facility is blocked after about one day of searching. The next day the search will not work anymore. Also radio-stations can not be selected. Earlier I had spinning circles. I changed my provider, I changed my router twice, I excluded my Apple gear, I connected to the Google-DNS-server. The only remedy is to restart my core.
@dylan has suggested several changes but so far the problem has not been solved. @support, please help me.

Pieter dVR