(I put this here as a mini HowTo as it might happen to others as well. A better place would be in the wiki).
I was also trying ESPeasy on one of my nodemcus but decided to set this one back on ESPimatic.
Flashed espimatic to nodemcu, disconnected/reconnected power, in AP mode I set my WAN config, disconnected/reconnected power.
Now I can access ESPimatic and the first thing to do is to upload all .js and .html files so I did. Somehow this did not function. I don’t know why, maybe a hickup in my WAN. I did not get a percentage complete counter for the upload of the files: nothing.
And nothing worked anymore.
So finally I disconnected/reconnected the power from the ESP to let it “reboot”.
After reboot I got a “corrupt” screen with no menu. I could not start http://<ip-address>/filemanager.html (or other html files) as obviously almost nothing had loaded but still Espimatic tried to present something, but actually it was completely non-functioning.
I finally solved it by issueing in the browser the command “http://<ip-address>/format”.
This did it. I could “reboot” the esp and start again. This time I could upload all files.