...I thought I disabled all of the non-working (white blank background) portraits, but there are still some. The problem is, I have no idea what these portraits are. Is there any way to determine it, so that I can disable them, too?...
One rather tedious way is to create a character with the blank portrait, then open its .bic file with GFF Editor to examine the portrait name.
Using the CEP 2.4 starter module, I see that there are actually two different problems; portraits whose icons are whitespace, and portraits with valid icons which appear as whitespace when selected.
An example of the former is po_dwghost_. This portrait doesn't exist, period.
The latter is far more common. It happens when a portrait exists, but the _h size variant is missing (a common shortcut when making portraits for NPCs).
In both cases, this can be corrected by flagging the portraits as "plot". (Alternatively, purists might want to correct the race in portraits.2da where this is clearly wrong, with the caveat that, for some reason, character creation seems to show portraits for more races than the one selected).
Since this is a CEP bug, you could report it on the
CEP wiki. I'm not sure whether the CEP team will have time to fix it before 2.6 is released, but it's worth a try.
Maybe a moneo script could be written to find the portraits that qualify for character generation that have no _h sized tga?