@Joram, 

I've encountered some operators who parse most of their error codes into the pesky "...invalid MMI..." for display on the handset (BTW the exact notification varies from one phone manufacturer to the next).

The exact spread of that 40% failure rate will give pointers to the nature of the problem. 

• Is it 40% of all attempts from the same handset? If so, there could be a resource problem with one of the signalling stack involved; particularly on very busy networks.
  
• Is it failing consistently for some select handsets, translating to the 40%? If so, check the Data Coding Scheme at play. The USSD-GW vendor may need to look for ways to maintain the DCS of the incoming traffic.
  

Either way, an on-the-wire trace (snoop/tcpdump/wireshark etc) between the communicating entities never lies - that's my mantra 

PS : You may need to check the content of the messages at play....most equipment vendors do not take raw special characters kindly.

Tony.

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