I haven't been able to find any info on this issue, but in the Fort Zeakden battle Delita is visually bugged. I have not edited him at all in any battle in ENTD, I have tried using different classes including his default and still the same visual bug. You can't see in the SS but his portrait is a female black mage for some reason and the SS shows what he looks like all the time here. He does not look like this anywhere else in the game, just here.
The event is trying to load too many sprites. The limit is 9, but since 5 are taken up by the party there can only be a certain number of unique enemy sprites.
In Vanilla:
Ramza, Delita, 3 party members, Algus, Zalbag, Female Black Mage, Male Knight is 9.
Odd because I didn't add or remove any units to/from this battle. Still has the same number as default. I guess I can remove one though if that will help.
I'm kinda curious too actually because you also have a female black mage in your party that shares a sprite with the enemy team...
Actually, the game reserves a certain amount of slots for each team.
Let's say you can have up to 4 units in your 1-2 squads, 4 slots will be reserved for your squad units, and the 5 remaining will be used for ENTD units. It will never use more. Even if you only pick Ramza for the fight, you will still be limited to 5 ENTD spritesheet slots.
Also, in pSX you can just press F12 to take screenshots. They are added to the screenshots folder inside your pSX folder.
When you changed the units that were on the map, did you change Golagros to the calculator/wizard sprite? That's probably where the sprite limit glitch is happening if you did not. Alternately, if any units that are hidden exceed the limit.