Most likely the file is locked by another process, or if the file is open RO in another process space and it may be that Sublime is being more fussy than other programs when it detects this to be the case.
It would be nice to see this investigated, but in the meantime, the workaround seems to be to save files under another filename/directory.
It would also be helpful to find a repeatable workflow to reproduce this error (haven’t seen it myself). Hopefully Jon will chime in then.
S