Hello,
I am not sure what you mean by “mark several lines”. Do you mean Bookmark, or Auto-Marker, or a simple selection?
If EmEditor crashes, a crash report file will be created usually in the C:\ProgramData\Emurasoft\EmEditor\Error folder. Please email the most recent file (compressed to a Zip archive) to tech @ emurasoft.com as an attachment with the detail information including procedure to reproduce your issue.
You can also provide a video to explain the issue.
Thank you,