The <FILE> was opened and contained too long lines (more than 1024 characters). Too long lines were wrapped and the document is set to read-only mode, as saving will modify its content. The solution is pretty simple.
Settings > Configure Editor > Open/Save
Line Length Limit to zero (0).
As the internal editor refuses to remember the setup after closing Krusader, replace this stupid editor with KWrite.
Settings > Configure Editor > General
At the bottom of the Viewer/Editor tab, replace the “internal editor” with “kwrite” (without quotes). Manually change the config file (using nano editor, or any other):
Dynamic Word Wrap=true Line Numbers=true Line Length Limit=0