Carrier 5-ton one-speed heat pump with 7-day 'stat, 3 years old.

Last night I noticed the house getting a bit stuffy, checked the 'stat. The system should have been running but was not. Switched the blower to ON, it came on. Went outside, the condensor was not running. *Some* of the lines inside had frosted over. There's a cylindrical (collection?) tank adjacent to the compressor. A line coming out the top of it into the side of the compressor (or vice versa, as the direction of flow may be), was one section that was frosted, but the collection tank itself was not, nor was the compressor.

Shut it all off for a few mins (should been long enough for the short-cycle timer to reset), then back on. Condensor wouldn't start. Raised the 'stat temporarily, waited a half-hour for the frost to melt, lowered the 'stat, and she all came on. I noticed the aforementioned section of line frosted up again prit'much immediately, but the house cooled and the system cycled, which it has been doing since then. However, feels like the humidity level is a little higher than usual.

Any opinions on what may have happened? Something needs to be checked out promptly, or wait/see if it happens again?

Last year had two PC boards in the blower unit go bad, one controlling the blower, the other for the heat strips. Dunno if the incidents were related. The installing service company did the blower repair. I was not particularly impressed with their service, so used a different company (also authorized Carrier dealer) for the 2nd repair. That service tech made a vague remark about these boards tending to be wonky and the design has been changed. If I'm gonna be hit with glitches/repairs every year already on a "new" system, that's not acceptable.

Filters were changed last week.