Does it work if you mount it with the host path (mount d /users/path.../elite3.iso -t iso)?
And does it work with an up to date SVN built for Windows?
And have you made sure that you are not doing anything in between that locks the file somehow (but that's just me grasping for straws 😉)?
I'll have to try and see if I can pin down when it went bad, if I can reproduce this (I'm keeping backups of my builds for easy regression checks).

I tried the literal path (the path shown in the working version after mounting) as well as the relative path (C:\z_ISO\image.whatever and image.whatever from within the directory), and neither worked. The example of it working with the older version shows it working as far as path and file locks are concerned, and I ruled that out right away by mounting it merrily all over the place from other systems. It's never been an issue previously, and I'm using all the same batch files that mount the images prior to whatever I'm doing, so nothing has changed outside of the DOSbox SVN version. Every once in a while I just replace the DOSbox application with updated versions.

I just tried on the Windows SVN both with literal and relative path with no issues.

Let me know if there is anything else I can try to help with.

That is weird. As it is an errormessage of a different program (not imgmount)
In fact, I would guess that a different program is being run. (LOADROM instead of IMGMOUNT)
Did you happen to copy IMGMOUNT of the Z:\ drive to a different drive and loading that ? (an old version of IMGMOUNT basicly)

I only have time to test in an hour or so, but please give us your complete commands you used in dosbox since you started it and try z:\imgmount instead of imgmount.

Edit: Qbix is probably right, you must have copied imgmount from an earlier version from z:\ to the folder you mount as c (or somewhere else in your path). I don't understand why one would do that but you better don't 😀
(I tried to reproduce imgmount problems with my snapshots and couldn't hence my agreement with Qbix)

So I figured it out. I have an actual copy of MS-DOS 6.22 that is set in PATH= to in the dosbox.conf, and somehow during some drunken revision I had omitted Z: from the path. As MS-DOS has it's own set of commands, including imgmount.com which is what is causing the confusion, I hadn't noticed as I rarely play CD games and everything else works correctly. After adding Z: back in, it works as expected.

Thanks for the help in tracking it down, it was driving me nuts.