@janulr I have found the same issue - unable to open M5F files after saving and closing. But looking at the clues above I found a workaround.
I have a custom block for DS18B20 in the original file. If I add the custom block back in to UIFlow again and then open the file IT WORKS. Otherwise I get a blank Blockly screen and most of the pyton code is missing, Display screen is also blank. Hope this helps.
Developers need to find a way of warning when the M5F is opened that a "Custom Block is Missing". That way we can load the block and continue.