There are 2 cases where this issue can appear:
1. The REST API fails with a TimeoutException (500) when the content length of a PromptSet Variable (input part in JSON) is larger then 2045 characters.
According to the documentation the UC_DATATYPE_STRING type is unlimited, however the when sending a too long variable content the JCP prints the following message:
20191115/092926.835 - 166151 U00045014 Exception 'java.util.concurrent.TimeoutException: "Wait for response of request 297 timed out after 60 seconds."' at 'com.automic.kernel.impl.CpDispatcherComponent.waitForResponse():108'.
20191115/092926.835 - 166151 U00045099 The server replied with following status: '500'
Release : v12.2, v12.3
Component : AUTOMATION ENGINE