In recent days pretty annoying UI bug appeared. There always is a tiny little think, which follows the pointer and partially hide the top-left corner of a letter, which is currently being typed. Concretely in my case it appears everywhere in the workbook.
The pop-up message says that some parsing error appeared.
I experienced the problem only in Safari 11.0.3. There seems no problem in Chrome 64.0.3282.140.