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It is the screenshot of the error I get.

(Click to enlarge)

When I add another page without letting redline appear during the edit process, I can have a clean page without redlines. I prepared the same pages without redlines now by starting over.

Oh that is funky. Looks like the WYSIWYG editor is converting that to actual HTML and making it part of the page. I'll investigate.

I'm not sure what exactly you did to cause this to happen but I cannot reproduce. Can you be more specific? It seems you did more than just add a page. Perhaps you used a certain key combination? Copied pasted something? Created blocks?

I uploaded some pictures to the media folders on Composr, and then I used those pictures in comcode boxes and put some links to some other pages.
I have been playing on the same page (editing, deleting and adding some new lines using "red line") yet, I wasn't able to reproduce the error, either.

I'm closing this as v10 has entered long-term maintenance status, and this is not a critical bug. Feel free to open a new tracker issue if you upgrade / use v11 and find this bug on v11 as well.