I've been seeing this X here and there, I marked them with a red box...

.

What do they mean and if they indicate a problem how do I fix it? They all seem to work right but I want to understand as much as possible.

It means that those brushes use a separate brush tip file and Krita can't find or load those brush tips.

ok, thanks. Is there a common reason/fix for this?

the reason is usually that you deleted the brush-tip that these brushes are looking for.

The fix is reassigning a brush-tip and saving them again, or actually installing the brush-tips.

Hi mperryman,
If you open resource folder you may notice that
Brush tips are the brush texture stored in the brushes folder, and Brush presets are stored in the paintoppresets folder as .kpp files.
As Wolthera says some how your brush textures or brush tips may be missing. If you got those brushes from the web look for the textures and just copy paste them back.
If you got those brushes as .Bundle ( new format for handling resources) files chances are that the author of those brush presets forgot to add the brush texture.

TheraHedwig wrote:the reason is usually that you deleted the brush-tip that these brushes are looking for.

The fix is reassigning a brush-tip and saving them again, or actually installing the brush-tips.

Hmmm, I don't usually delete files unless I know what they are or I put them there. I especially don't delete things in progs that I'm still new to. It must have been something weird. And yes, they were bundles, I'll reinstall them.