The red background is probably an indication that the error is being generated by the invoiceplane application,
Please check your .htaccess file first. You can use this as a quide
https://wiki.invoiceplane.com/en/1.5/getting-started/installation#subdir
If yous installation is correct, then I would suggest changing the debig settings in ipconfig.php file and looking for the log files in the logs directory
https://community2.invoiceplane.com/t/topic/6905