Ha ha ha, don't you just love internet forums? I couldn't get the trolls up under power, either of them. I was thinking the left one's solenoid power may be broken, but the fact that neither would rise in T20 meant that was not indicative of anything. I followed the instructions in that manual text though, and cycled both trolls a few times, raising them with latext clad hand and then banging their faces a bit. The switches were working perfectly, but I was trying to "reset" the trolls somehow as described in the text.
Anyway, then I rebooted the machine, same L Troll Switch bad test error. I made the trolls come up in the game just to check, and was gobsmacked when they both rose. So that's good news, sort of.
Switching the machine off, waiting, then back on again shows that the self test now runs without error.
Conclusion: you can get some weird troll error which you can clear by manually "testing" the trolls as in T.20