Steps to reproduce

Create an XML-RPC client and send the following request to the server. This request contains 4 params, the fourth one being an associative array (struct in XML-RPC).

<?xml version="1.0" encoding="UTF-8"?>
<methodCall>
<methodName>set_participant_properties</methodName>
<params>
<param>
<value>
<string>ZTF~bMZs6aE~BuQW1l5MVCpn0~gNFccC</string>
</value>
</param>
<param>
<value>
<int>366661</int>
</value>
</param>
<param>
<value>
<int>11</int>
</value>
</param>
<param>
<value>
<struct>
<member>
<name>email</name>
<value>
string>1707321300349@gmail.es</string
</value>
</member>
<member>
<name>lastname</name>
<value>
<string>1707321300349</string>
</value>
</member>
<member>
<name>firstname</name>
<value>
<string>1707321300349</string>
</value>
</member>
</struct>
</value>
</param>
</params>
</methodCall>

Expected result

The previous should work as the method expects an associative array as forth parameters. Limesurvey tests are programmed with this in mind and the pass. It also works with JSON-RPC.

Actual result

We receive the following XML with the error: "Calling parameters do not match signature"

<?xml version="1.0" encoding="UTF-8"?>
<methodResponse>
<fault>
<value>
<struct>
<member>
<name>faultCode</name>
<value>
<int>623</int>
</value>
</member>
<member>
<name>faultString</name>
<value>
<string>Calling parameters do not match signature</string>
</value>
</member>
</struct>
</value>
</fault>
</methodResponse>

I am going to send a pull request in Github that solves the issue.
In the attached document I explain where the problem is and how it is solved.