The upload didn’t work.
First try this out:
https://community2.invoiceplane.com/t/topic/10011/36
and this:
https://wiki.invoiceplane.com/en/1.6/getting-started/updating-ip#1511-16-instructions
and this:
https://community2.invoiceplane.com/t/topic/10324/4
With an internal server error, the error should at least end up in the server log. Check there as well.
Most likely the solution is to comment out that first line in ipconfig.php (see the documentation link)