When an image caption on the first line of a page contains the string "page " (including the trailing space) followed by any text the right half of the VisualEditor toolbar disappears (everything right of "more"). This includes the save button so edits cannot be saved (hence the "major" severity)
This is likely related to either bug 54379 (same symptoms) or bug 53312 (same trigger).
Comments on the VE/F report at https://en.wikipedia.org/w/index.php?title=Wikipedia:VisualEditor/Feedback&oldid=574608525#Odd_one suggest this might be restricted to FireFox.
Minimal test: https://en.wikipedia.org/w/index.php?title=User:Thryduulf/sandbox4&oldid=574608905
Real-world example: https://en.wikipedia.org/w/index.php?title=Clarke,_Irwin_%26_Company&oldid=574526057
- Create a page with any image on the first line of page
- Give the image a caption containing the string "page " (including the trailing space) followed by any text.
- Save the page
- Load the page in VE and observe the right half of the editing toolbar.