All your requests in your browser are mixing up https and http. Until that problem is resolved (no more red errors in your Network tab of your browser) you will keep having thsw problems
can you please send to me in private (use the messaging function in this forum) the ip_config.php file (please delete any passwords / private keys before)? There must be an error in the file or while the system is trying to read the file. This can also be seen in the screenshots you sent us, because the “guest url” field in the invoices is an HTTP url, although you are visiting your website over HTTPS.
When using InvoicePlane over the HTTPS protocol (this usage is encouraged!) please check that the IP_URL variable in the ip_config.php file also starts with https://.