[TSVN] tortoisemerge does not save files when accessed by UNC

I browse my working copies using the unc name in windows explorer (i.e.
\\server\path\workingCopy not a mapped network drive)

When using tortoisemerge to view differences or resolve conflicts, I can
'use this text block' to modify my working copy on the screen, but when
I come to save the changes a dialog box comes up.

This dialog has an image of a red circle with a white cross, and an OK
button.

Unfortunately the changes I made are not saved to disc, even though the
save button becomes greyed out when I click OK to this unhelpful dialog.

I would expect the save button to have saved the file, but this dialog
comes up and the changes remain unsaved.

The same thing happens when I close tortoisemerge when it still has
unsaved changes and the 'Do you want to save changes' box pops up.

I am using tortoiseSVN 1.2.0 build 3602 on windows XP pro SP 2

Please can you please cc any replies to me.
Pete.