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I suspect I know the answer to this but here goes ...
HT2 BB with standard 24mm cranks (Raceface Evolve). Always been fitted with correct torque on the retaining bolts. No side to side movement in the crank arms when I fit them but after a ride or two, say eight hours riding or so, there's definite movement in the arms.
I've checked the bearings and there's no discernable off-axis movement so are the cranks on the way out? They are five years old.
The plastic/rubber spacers that RF cranks come with do compress and degrade over time, so I'd replace them (or add another one). Might solve the problem.
My RF cranks started dying when the threads for the connecting bolt thing went, replaced with a new one for a tenner and that lasted about a year, then completely went - couldn’t tighten, only the lock ring kept them from coming off mid ride. So changed for another 24mm crankset (went Shimano Zee as on offer at Crc)
Actually, mistake on my part, they are Shimano cranks. So battered I couldn't tell! I don't know when they got fitted as the original parts list has Raceface, perhaps when the original Raceface BB died.