Theme editor (customize) blocked ... Shopify refuses connection

[Digit_PaxMentis]

Hello ... for my collection overview page I have created a template, which allows me to add individually the collections I like to show to my visitors - see https://www.pomshop.nl/collections (without info text) and https://www.pomshop.nl/collections/the-pom-collections-overview (with info text).

I got a new Podcast collection now https://www.pomshop.nl/collections/pom-video-podcast-collection and I like to add this collection to specific template pages (e.g. I write blog posts related to a new podcast, so I also like to add the podcast collection in the footer via a specific template).

I have to get access to these templates via the Theme editor ("customize button"). The URL after clicking the "customize button" is https://admin.shopify.com/store/ID-No/themes/12digitnumber/editor  

Yet, today this access was resisted. Only a blank page with the text info "IDno.myshopify.com refused to connect"

And then just pops up a warning window:

Failed to load the page

This page is not compatible with the editor due to one of the following issues:

• Page is redirecting to an unsupported URL
• Page failed to load due to an error

What I did first ? Clicking the "Try again" button of the pop-up window does not work, and visiting the documentation page ( https://help.shopify.com/en/manual/online-store/themes/theme-support/troubleshooting ) is not helpful.

1. I have tested the shopify store, yet there is no log about any issues. "green hook", all seems up. And also no events are listed. 

2. Then I have downloaded and installed parallel the latest version 15.02 of the Theme "Taste" which I use and same here, I don't have access to the editor for customization. The same and only hint I get is "IDno.myshopify.com refused to connect"

What to do ???