I've got a serious problem with 645Z + Metz's flagship unit (Metz 64 AF-1 digital). Namely, 2 units already have died on me when used on the 645Z. The first unit was bought Dec.19.2014 in the Czech Republic (Prague). It was set in P-TTL mode with the secondary flash at 1/2. After 15 or so shots the first day, with the same setup the flash exploded (the bulb in the secondary flash died with a bang). Apart from visible shattered glass inside the secondary flash there aren't any defects. The flash won't turn on however.
At first I thought it's an isolated incident, informed Metz and they've told me to handle it with the dealer in Prague (30E fast shipping back to Prague). Meanwhile I've ordered a secondary unit from HK. Same scenario. 15 shots or thereabout the first day then after a few days not being used, the flash died on the first shot today (secondary bulb explodes with a loud bang with shattered glass inside).
Pic:
I've mailed Metz again, and trying to sort things out on Ebay with the seller (the HK unit). Meanwhile, if you do have any suggestion share it here. Metz informed me they never tested the unit with the 645Z but that's a silly excuse.
My suggestion would be to AVOID using the secondary flash on your Z.