Create new client API degradation
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## Summary
After upgrade to 3.3.1 the creation of new client stopped working with the following error:
ISPConfig SoapFault(data_processing_error): usertheme_snytax_incorrect<br />
## Steps to reproduce
1. Use the api to create a new client without populating the 'usertheme' field like so:
```
$res = $this->call('client_add',
Array('session_id' => $this->session_id,
'reseller_id' => 0,
'params' =>
Array(
'username' => substr($fullusername, 0, 25),
'customer_no' => $fullusername,
'contact_name' => $fullusername,
'email' => '***masked***',
'password' => $password,
'web_php_options' => array('mod', 'php-fpm'),
'ssh_chroot' => 'jailkit',
'country' => 'BG',
'language' => 'en',
'template_master' => $template['template_id'],
'limit_web_domain' => 1,
'limit_web_quota' => $template['limit_web_quota'],
'limit_database' => 1,
'limit_cron' => 0,
'limit_cron_type' => 'chrooted',
'limit_cron_frequency' => 1,
)));
```
$this->call is just a wrapper for the soap call
This code has not been changes since 2019 and it was working fine
## Correct behaviour
<!-- What should happen instead? -->
It should have continue to work after the upgrade
## Environment
Server OS + version: (Debian 12) \
ISPConfig version: (3.3.1)
<!-- _you can use `grep 'ISPC_APP_VERSION' /usr/local/ispconfig/server/lib/config.inc.php` to get it from the command line_ -->
Software version of the related software:
<!-- You can use 'nginx -v' or 'apachectl -v' to find the webserver version. Use 'php -v' to find the PHP version.> Put this in code blocks, like so: -->
```
Server version: Apache/2.4.66 (Debian)
Server built: 2025-12-05T18:54:44
PHP 8.1.34 (cli) (built: Feb 13 2026 15:51:52) (NTS)
Copyright (c) The PHP Group
Zend Engine v4.1.34, Copyright (c) Zend Technologies
with SourceGuardian v16.0.0, Copyright (c) 2000-2025, by SourceGuardian Ltd.
with Zend OPcache v8.1.34, Copyright (c), by Zend Technologies
```
issue