Ok, I broke out the 3.3V RS485 chip I have (an LTC1480) from the “someday I’ll test this” box. I’d bought the chip ages ago to try out in place of the (5V) MAX485 that I couldn’t ever get to work. But since the automatic flow control always worked nicely, I threw the LT1480 onto a shelf unopened.
Anyway, it worked for me. I made some very small tweaks to the SensorModbusMaster library to see if I could get the timing better, which helped a little, but it worked even without. I didn’t have any issues with the board restarting or anything.
Hm.. In my test I used AltSoftSerial (pins 5&6) as my serial and tried the RE_/DE on both pins 4 and A2. What exactly are your connections and what code exactly are you running?