I finally got it working again!
I did a full fresh installation on a new SD card with complete latest Raspian and new MM and all the modules I used before…and still the same behavior: MMM-IFrame showed nothing and I got the same warnings as mentioned below.
Then I tried MMM-SmartWebDisplay by @AgP42 which finally displays my video stream again. (BTW: This also works with other URLs as described in the documentation)
Many thanks for this module.
For all having issues with MMM-iFrame, MMM-IFrame-Ping etc., try MMM-SmartWebDisplay.
Don’t forget to do a NPM install in the module folder once the module is cloned.