I installed the VMInvoice Autorun plugin without a problem. I however run into the following error message when I activate the plugin:

Warning: file_put_contents(.../tmp/vminvoiceautorun.log) [function.file-put-contents]: failed to open stream: Permission denied in .../plugins/system/vminvoiceautorun.php on line 81

This message then appears on all admin pages (making the font to appear bigger than normal) and the only way that I am able to get rid of it is to inactivate the plugin once more. How do I resolve this problem?

Hello,

VM Invoice plugin stores a logfile to Joomla's tmp directory, which is unwriteable in your case.
So make sure the directory you defined as temporary in your Joomla is writeable.