Workaround to load old m5f files:
Open a m5f in a text editor, save-as with a new name, and insert the below after
"Blockly.Remotes":[],
in the end of the file:
"Blockly.RemotePlus":[{"createTime":1625241945254,"name":"M5Remote","dragAndDrop":false,"resizable":false,"color":"#fff","bgColor":"#0080FF","type":"title","id":"__title"}],