It looks like a bug in the LABEL command to me. It'still the same under VFP8beta, BTW. You can try following workarounds:

1. Every time before previewing, copy your label file mylabel.lbx to the temporary file and run from it.
2. Try (NOT TESTED) REPORT FORM mylabel.lbx PREVIEW

>We have a project with a number of reports and labels. We do not build either reports or labels into the exe, so that users can customise them.
>
>We have discovered that when more than one network user tries to preview labels based on the same lbx file, a 1705 error is generated. This does not happen with reports.
>
>We have tried creating completely clean labels to ensure that there is no hidden code (eg exclusives in the data environment). The error still occurs. Any ideas?