AT command interface works! The cause of the breakage was the same Nucleus API issue with NU_Create_Timer() which we encountered at the very beginning of this project with Riviera timers: the code in uartfax.c from TCS211 was passing 0 as the initial dummy value for the timer duration, and our FreeNucleus version doesn't like it. The fix is the same: pass 1 as the initial dummy value instead.

#define	MAX_FN_COMPONENT	20
#define	MAX_DIR_NEST		6
#define	PATHNAME_BUF_SIZE	((MAX_FN_COMPONENT+1) * (MAX_DIR_NEST+1) + 1)