It's due to this fix in r80:
ebStorage: expressions that return stored values now type-checked as returning strings. WebStorage always returned values as strings, but previously the WebStorage expressions could be used where a number was needed.
You used a WebStorage value as a number somewhere, and it's actually a string, and since r80 the editor recognises as a string which prevents your project opening. I fixed it by wrapping the WebStorage values in int() in the XML files. Recovered project