It works fine for me, so it doesn't seem to be a general problem. But of course, that doesn't help you.

Let's start with the blank preview. Make sure the sample width is set to something sensible. Often it defaults to something so small that it only shows a tiny section of your pattern, which may be a blank area. Since your pattern is 20x20, try something like 100. Not that this is really important, it is just a preview after all, what matters is if it works when you try to use it in the map, which you didn't mention at all if that was a problem or not.

Now, as for the crashing. That dialog can sometimes be a bit fickle when you add/remove styles. My general recommendation is always make everything ready, which in your case includes defining the symbol, then save the map. This gives you a read to use starting point in case of a crash. It is also possible to do the fill definition in steps, i.e. go in first, create a new symbol fill, then immediately close the dialog and save. Then go back in, and change the symbol for your new style to the one you made. This makes it easier to track down exactly what causes the crash if it is repeatable.

If you attach your map here with the symbol to use for the fill already defined I can also try to define it, see if everything works fine for me with your map and symbol, or if there is an issue with either.

Keep in mind that the names of the fills doesn't mean anything, it is just a name. Just because that particular fill happens to be named "background" doesn't mean it is the fill that is used for the map background.

If you look at my screenshot below, I have used the List command on the background polygon. In the list output, you can see the fill is "P3B Parchment Background Bitmap", which you can then look up in the fill style dialog as I did in my screenshot.

As Steven chimed in above, this comes from Perspectives 3. (It can be a bit difficult to read from the fill style file name since it doesn't mention Perspectives int he path, but P3B means Perspectives 3, bitmap style B.)

Those "C:\CC2\" missing files are unlikely to be an issue, they're probably just some remnant definitions from old times that still is defined, but not used. I doubt Steven would have those either. Same with those "E:\FCW32" references. Sometimes some of those old references decides to hide out in the list of fill style, but it doesn't matter if nothing in the map actually references them.

If you see a red X in the map, use List on it. If it is a symbol reference, it doesn't matter what fill style it has, as you are dealing with missing art for the symbol itself, which should then be checked by opening up the symbol manager, finding the missing symbol in the list there, and hit the list button to get info on the paths of the symbol artwork files, while on the other hand if it is some kind of filled polygon, then the fill style matters, and you can look up the name in the fill style dialog and check the path.

Note that the file structure for CC3+ is exactly the same as for CC3, with one critical difference. While CC3 kept all the data files in it's installation directory along the actual program files, CC3+ instead puts all the data files in a separate directory, C:\ProgramData\Profantasy\CC3Plus by default, insteaf of the installation directory (C:\Program Files (x86)\Profantasy\CC3Plus). You'll find everything you used to find in the installation directory in the data directory.

No, opening and saving doesn't matter. CC3+ doesn't care if you work on a map with a font that currently isn't installed. The text entity will still remember what font it was defined with originally (or changed to if you use change properties on the entity, obviously). But the visual replacement font is just a visual thing when it can't find the font, and it will display the text using Arial instead of the actual font.

Are you sure that that Calligraphic font isn't what you are after? If you don't have it installed, maps will show using Arial, like in my screenshot above, but CC3+ will still list it as the actual font. (Just as in my example above, the text is clearly plain arial, but listed as XXII Arabian-Onenightstand, a font I don't currently have installed, which makes it look very plain indeed until you consider that isn't the font used in the display because I don't have it)