Couple of things to check:
Make sure you have .NET framework from Microsoft (silly browser launched game)
Graphics drivers up-to-date?
Don't know what exact error you were having, but those issues are common on the forum.
@All of the above, yeah commando is pretty terrible right now. I actually have a lv. 22 nat comm. with a K/D ratio over 2 that I'm playing as little as possible right now, as I know the sight of my ratio plummeting to under 1 will depress me. Instead I fool around with a level 9 Royal comm. and have learnt that the long pistol/knife isn't too bad (but it may be due to my purposely bad ELO and low level I'm being matched up with idiots).
As for soldiers, they're pretty alright at the moment. Devs hinted that they will get a deserved boost in mid-range damage. I'm doing okay-ish with mine right now. If I hang around with a gunner/soldier it makes things 10 x easier.
Gunner after the boost is a lot of fun, but not much fun to come up against. I don't care how well I do with mine, I just love rocket jumping/spamming and kegging groups of enemies while I (try to) mow them down.