I wear my HUD, or login without being gagged, and I am blocked from IM. When I detach my HUD, I can IM again.

There is a known bug up to HUD v4.12.02 and Signature Gag v1.05.05, if you wear your HUD or login (while ungagged) with the garble setting at "loose".

The immediate workaround is to rez the gag, and change setting to "tight".

This is fixed in HUD v4.13.00 and Signature Gag v1.05.06 and after.