Export PNG weirdness

[Chris Carroll-Davis]

Hello everyone.

Can anybody help with this ...

I am exporting PNGs from fields, using this syntax:

   export snapshot from field fname to file imgPath as PNG

This works just fine on about 30 or so fields, but when I come to one  
particular field it fails with error message:

     export: can't write to file, mask file, or container

All the fields are opaque.  If I switch this field to non-opaque...  
it works fine... or if I export as JPEG it works fine!  The only  
difference (that I can see) between this field and others is that it  
is larger than the others.

Has anyone come across this problem before?  Could there be some  
other property that I have overlooked that is causing the problem?

I am pulling my hair out at this one (and at my age I don't have too  
much to lose)


Chris