I want to chime in here. I just installed this tonight and the error appears to be coming from the plugin trying to save the settings in the Business Process tab/section.
I reviewed their forum and found 1 topic that was posted last year in regards to the issue. I posted my comments as well.
I own my server and host sites off it so I went in directly to my apache logs after I got the error from WP-Invoice (upon trying to save from only the Business Process page). The following were my results:
ModSecurity: Access denied with code 406 (phase 2). Pattern match “long line of code” at REQUEST_HEADERS:Cookie. [file "usr/local/apache/conf/modsec2.user.conf"] [line "98"] [msg "SQL Injection Attack"] [severity "CRITICAL"] [tag "WEB_ATTACK/SQL_INJECTION"] {hostname “www.mywebsite.com”] {uri “/wp-admin/admin.php”]
It appears my server is stopping the WP Invoice plugin (or I’m assuming the Invoice plugin) while trying to save something that the server is considering a Web SQL Injection Attack. Yay server security!
I’m waiting for a response on their official website regarding this. Jayseventwo and klynam, since you’re also having the same issue, it could be the same problem. Do you have access to your apache logs right after getting the error?