Further testing revealed that the problem seems independent of the “BB” (NCO) frequency. The problem seems to occur whenever the “RF” frequency is close to 30 MHz and the bandwidth is increased (which just happens automatically when you use the “SoapySDRDevice_setFrequency” C-call with a target frequency below 30 MHz.
If I manually set the RF and BB frequencies and if I set the bandwidth only after activating the stream, then there is no error, but I assume the calibration will be wrong then. 
Does anyone know what’s the right way to initialize the LimeSDR Mini with SoapySDR in C? Where can I find documentation on this? Can anyone else reproduce my problems with an “RF” frequency close to 30 MHz and an increased TX bandwidth?