Import email configuration from ISPConfig 3 - Internal Server Error

I'm trying to import the email configuration with the tool from the web interface.
I've configured a remote user in my old server, given him all the rights.
I then go to the ISPConfig 3 mail function, put in the details, but when I click Connect to remote server, I just get an "Internal Server Error" message come back.
I have valid certificates on the old and new ISPConfig servers. Old server running 3.1.15p2 and new server running 3.1.15p3.
Are these errors logged somewhere to help debugging? I've enabled debug but nothing relevant appears in /var/log/ispconfig/ispconfig.log - maybe I'm looking in the wrong place?

 

Thanks @till - I looked, but nothing.
I've rebuilt my server using Debian 10 and Apache (gave up on nginx because of the mod_rewrite issues).
I've looked in /var/log/apache2/error.log but nothing is added when I click on the Connect button

 

Ahh, I feel silly. I hadn't thought that "Internal Server Error" would be a message from the remote server!
It looks like I'm missing the Soap Server Now to find how to install that

 

Hmm, I've done that, but still get an error message

Code:

[Tue May 26 17:31:13.726464 2020] [fcgid:warn] [pid 9018] [client xxx.xxx.xxx.xxx:34204] mod_fcgid: stderr: PHP Fatal error:  Uncaught Error: Class 'SoapServer' not found in /usr/local/ispconfig/interface/web/remote/index.php:16
[Tue May 26 17:31:13.726496 2020] [fcgid:warn] [pid 9018] [client xxx.xxx.xxx.xxx:34204] mod_fcgid: stderr: Stack trace:
[Tue May 26 17:31:13.726500 2020] [fcgid:warn] [pid 9018] [client xxx.xxx.xxx.xxx:34204] mod_fcgid: stderr: #0 {main}
[Tue May 26 17:31:13.726503 2020] [fcgid:warn] [pid 9018] [client xxx.xxx.xxx.xxx:34204] mod_fcgid: stderr:   thrown in /usr/local/ispconfig/interface/web/remote/index.php on line 16

As this is an older server that's been updated, how can I confirm which version of php the remote part uses? I have 7.2 as the default, but wonder if ISPConfig is using another version.

 

And of course, I was looking at the wrong server again.
The logs are on the source / remote server
The php-soap needs to be installed on the target / local server
Thanks for your help