Hello there!
So I added InvoicePlane 1.5.9 to my subdomain (clients.emildeveloping.com) and I get the 404 Page Not Found page with red background.
I configured ipconfig.php to look like this:
IP_URL=http://clients.emildeveloping.com
… but still dosen’t work.
I’ve read on the FAQ and Wiki but haven’t found any solution for it yet 
Best Regards
Emil
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