Though I have no experience with PSP, I'm very sure that Fallout would need a higher Ms-Dos version than 5. I don't know if you can edit DosBox options file, dosbox-x.xx.conf, but if you can, try putting the following line at the very end under
[autoexec] section:
VER SET 7 10
That will set the Dos version to 7.10, which is what it was under Windows 98. Might try
VER SET 6 22
as well, since that was the last "Windows-less" Ms-Dos version I think.
Also have a look under the
[dos] section at the bottom and whether you have
XMS=,
SMS= and
UMB= values all set to
true. If yes, it might be worthwhile to set each individually to
false and see if that changes anything.
Just a shot in the dark at the possible realistic reasons here, since I have no experience with running DosBox on PSP.
Edit: Thinking on it, it also indeed might have to do with loading / writing only 10 files. Ms-Dos actually allowed more than 8 files, which was the minimum, through a
files=x command which was used back in Ms-Dos by a
config.sys file, loaded at the very startup. It allowed more files to load at a time, dunno what the maximum was, maybe 255. Reasonable amount back in these days was 20 or 30 files, also depending on maximum RAM available.
Don't think DosBox supports anything like that, though, at least not by default.
Have a look on those Dos parameters here, if you like:
https://www.computerhope.com/ac.htm