A6 GSM + leonardo = list SMS problem

Hi there,

I'm trying to communicate with the Ai Thinker A6 GSM module, which works quite fine until I want to list all received SMS from the SIM.

I'm using a arduino leonardo, A6 GSM connected on Serial1. When I try to list all SMS with 'AT+CMGL="ALL"' the leonardo returns me this:

There is some garbage at the end and no "ok". The same command run with a FTDI USB adapter works quite well:

+CMGL: 1,"REC READ","+49XXX",,"2017/07/04,21:26:28+02"

The interesting part: I had to raise the Serial1 timeout because the A6 needs longer than the default 1000ms to respond to this command: Different timeouts result in different garbage at the end. Already tried timeouts to 10s, with no luck.

I don't know what's the problem here.