It seems to be a problem with the rich text editor.
If you quote a post, then click "Source" above the edit window, you'll see some entries like:
Code:
[img]https://www.flyertalk.com/forum/undefined[/img]
A workaround is to go into your control panel, select "Edit Options" and change the message editor to "Basic Editor". But this means you'll be editing using bbcode.