I have had this issue with a reverse proxy that is misconfigured. Since you don’t mention having a proxy, I recommend looking for whatever changed. If this happened all of the sudden, then something must have changed. Do you have any backups of your invoice plane folder or your server configs to compare?
I am thinking that it could be related to the web server configuration. You mentioned setting this up for your client so double check that you’re initial setup has not been altered in some way. Perhaps a software update replaced a config file. Or an employee with good intentions tried to “improve” the server and it didn’t work quite right. Just a thought
I’m curious about your solution. Let me know if you make any progress