I would be suspicious of the contacts as well, both on the camera and flash/triggers; the A900 might be (very likely) similar to my A700 in the shoe area which has a tendency to self insulate the contact pin/pins with plastic transference from the action of sliding the flash on and off.
When I can make the flash icon and exposure line on the rear screen disappear and reappear just by moving the flash slightly I know it’s time to clean the contacts again.
The thing I’m suspicious of in your case is the ‘camera error’ message, I have not had that appear on the A700 but I suppose it may depend on which pin (or combination) of the four is not making contact….maybe.
I first noticed the problem with the A700 when it began producing either no flash for the exposure or full power flash over exposures, in other words no regulation at all.
A clean of the contacts both on the camera and flash fixed (I had to use something abrasive to get rid of the problem, first time it was CD buff/polish, last time I used microfine WD) that immediately and it was back to normal with near perfect flash exposures once again.
This might not be the cause of the problem in your case of course, from what I’ve noticed across the web when a ‘camera error’ message appears there is usually something mechanical responsible, and unfortunately the most likely suspect is the shutter/mirror/aperture control area. (Not a forgone conclusion but possible)
I guess you could check the AF by selecting in the menu item Release priority = release instead of AF to see if the error goes away, but I doubt that the auto focus would be the first thing to fail, you also could check the lens contacts/compatibility area with the menu item Release w/o Lens = Enable, but I don’t think there would be any problem in the lens area with the lenses you are using either.
As you are using the camera commercially you probably need to talk to Sony support and arrange for it to be checked and fixed, you will need to use a backup camera in the meantime of course.
Ps I’ll enclose a couple of diagrams from the A700 service manual (can’t find an A900 service manual), the items depicted are most likely similar in both cameras. As can be seen just replacing the shutter is a major job with almost a full procedural disassembly being involved with many items that are ‘use once’ insulators, isolators, joints and mountings that usually have to be discarded and replaced in the process.
A700 Iris Ring.jpg [ 90.06 KiB | Viewed 743 times ]
A700 shutter replacement.jpg [ 122.17 KiB | Viewed 743 times ]