It is certainly not normal behavior for CC3+ to intermittently show red X'es for images that worked earlier, but there are a few external factors I can think of:

• If you have the CC3+ data directory on an external drive like Thescabi mentions, it may be an issue with the drive or connection to the drive. Could also happen with an internal drive if the drive itself was bad.
• Your antivirus is a bit too greedy. Antivirus software often scan files that are opened, and while it does that, it denies other software access. If it keeps the file locked for too long, CC3+'s request may time out, which means it fails to load the file. If this is the case, just hitting :CC2RESMODE: and clear the bitmap cache, check "redraw on ok" and then hit OK should force CC3+ to reload all bitmaps.
• Either the actual bitmap on disk the drawing refers to have been moved, OR if the image is set up with a map-relative path, and you moved the map file but not the corresponding files (The latter is really only a possibility witch custom artwork though, not with the standard symbols and fills from CC3+ and add-ons). You already mentioned you didn't move things, just kept this point here to have a complete list.

Well, that's the main idea behind making maps, isn't it?

A map doesn't have to be some sort of artistic masterpiece to be a great map, a map should fit the intended use case. Visually stunning doesn't always make for a good map.

Thanks Jim.

Maps are now in the atlas. Thanks for contributing.

It is a bit difficult to see the end result. Could you make it a bit more visible to be closer to the visibility of the paving on the pier?

This happens if in Tools -> Options, you deselect the "Select with right button context menu" option.

The reason it pops up when you click in your map is that clicking in your map without a command active will automatically repeat the last command, and if the option mentioned above is unset, this will pop up if the command require a selection.