Hello Christian,
I don't think there is much relation between inbox call and UIF_CALLBACK -- actually, so far, I don't see there is any relation between them.
UIF_CALLBACK is a service whose handler class is CL_CRM_WEB_UTILITY.
Normally, with request for service UIF_CALLBACK, there should be a parameter crm_handler=XXXX.
XXXX is class name. When processing this request, in method CL_CRM_WEB_UTILITY->IF_HTTP_EXTENSION~HANDLE_REQUEST, the handler class 'XXXX' will be initiated into an instance and its method XXXX->handle_request will be executed to deal with paramters transported together with crm_handler class.
In CRM system, there are several places using this service.
But when I made stad traces for going around in INBOX, I didn't get UIF_CALLBACK after it. I also made a large search in INBOX and the 'waiting wheel' appears. But in STAD, I still don't see UIF_CALLBACK.
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Maybe we are checking different CRM versions? What is your version of CRM component BBPCRM and WEBCUIF? Or, maybe UIF_CALLBACK is triggered by other places? Can you find out what is the parameter for crm_handler?
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If this request takes time, maybe the time is spent in crm backend system. If so, you can make a ST12 trace for the specific 'username', tasktype is 'H HTTP', and then check for a specific operation, how long it takes, compare it with the UIF_CALLBACK you observed in ST05N. But firstly, you need to know exactly what operation it is, which operation triggers the UIF_CALLBACK after the BSPWDICCMP_INBOX entries.
Best Regards, Corrine