[Plugin: Magic Fields] Errors when Exporting a write panel

Hi

Can somebody tell me why I get several lines of errors like the following when trying to export a write panel:

Warning: fopen(/tmpinv.pnl) [function.fopen]: failed to open stream: Permission denied in /home/……….

Warning: fwrite(): supplied argument is not a valid stream resource in /home/…….

Warning: Cannot modify header information – headers already sent by (output started at /home/….

thanks