I build it last week (linux Debian): NetRadiant 1.5.0-svn169
Unfortunately, I can't reproduce it at will (just tried to start a new map and try lot of combination without success).
But with a whole map, it happened to me often (around 10-15 times after one week I use NetRadiant).
Sometime I got segfault just after clicking on "regroup".
On one of my testbox it happened everytime I forgot to hit Ctrl-Alt E (selecting the whole group) before trying to regroup.
(on a new map, it used to move brushes into worldspawn when I forget to hit ctrl-alt E).
Sorry, I can't help you more...