And even in 2022 people are very happy to find this information! Took me about 2 days to figure out what was going on. I did not get any compilation errors and uploading the sketch to the board went fine. However, even the cleanest MySensors sketch did not work on the Wemos D1 mini version 3.0.0. Other sketches like "blink" without Mysensors.h did work.

Also for me the downgrading to 2.7.4 works. Maybe an idea to put this info on the Mysensors page?

Thanks again for posting this solution!