Several errors since moving to cloud server

Since moving to a could server with Vultr, I get several errors popping up.
Like this:

A PHP Error was encountered

Severity: 8192
Message: strpos(): Non-string needles will be interpreted as strings in the future. Use an explicit chr() call to preserve the current behavior
Filename: MX/Router.php
Line Number: 224
Backtrace:
File: /var/www/vhosts//public_html/ip/application/third_party/MX/Router.php
Line: 224
Function: strpos
File: /var/www/vhosts/
/public_html/ip/application/third_party/MX/Router.php
Line: 71
Function: set_class
File: /var/www/vhosts/_____/public_html/ip/index.php
Line: 325
Function: require_once

Not sure how this is connected with Invoiceplane but I can’t issue any invoices anymore. I can view, yes but can’t actually do anything

I run on PHP 7.3.18

Thanks

Oh my goodness. Seem to totally fail.
Application always hangs but then it’s done anyway.
Sending invoice doesn’t work neither.
There’s a machine gun round of PHP shooting at me:

A PHP Error was encountered

Severity: 8192
Message: strpos(): Non-string needles will be interpreted as strings in the future. Use an explicit chr() call to preserve the current behavior

A PHP Error was encountered

Severity: 8192
Message: filter_var(): explicit use of FILTER_FLAG_SCHEME_REQUIRED and FILTER_FLAG_HOST_REQUIRED is deprecated
Filename: src/PHPMailer.php

A PHP Error was encountered

Severity: Warning
Message: Cannot modify header information - headers already sent by (output started at /var/www/vhosts///vendor/codeigniter/framework/system/core/Exceptions.php:271)
Filename: helpers/url_helper.php

Funny thing is, all my online forms but work perfectly, hence there can’t be something wrong with PHPMailer itself as the errors indicate. Weird.