Just to add another data point, I always used to just press F10[*] to remove the counters so I can look at the map when counter density is high, then click somewhere to bring the counters back, but since using VASL 6.5.0, it can take many 10's of clicks to bring the counters back. I sometimes see red lines flash on-screen (presumably a LOS check), and (I think) my opponent gets console messages saying that I've checked LOS.
We've also had one hang, I think because we both checked LOS at the same time, although IIRC, older versions of VASL did this as well.
[*] I know about Ctrl-F10, but I'm a keyboard user and this is an awkward two-key chord, and it's not doing my RSI any good :-/