I just wanted to provide an update on this issue with the solution, in case someone found it useful in the future:
After extensive testing and troubleshooting with two render farms (rebus and ranch), it turned out that the problem was in the glass material of the windows!!
Initially the windows had a CoronaPhysicalMtl, with the "glass" preset. For some strange reason, whereas the frames in my workstation were rendered fine, the frames rendered on the render farm nodes came out with flawed GI and caused flickering. This happened with both a pre-calculated UHD and with UHD calculated from scratch for each frame. However, it did not occur when PT+PT was used for the GI.
After changing the glass material to a CoronaLegacyMtl with "standard" glass settings, the Pt+UHD setting for the GI produced correct results on the nodes of both render farms. The same happened when I used a CoronaRaySwitchMtl for the glass, with CoronaPhysicalMtl on all nodes except GI (which was left blank).
Maybe this is some kind of bug for the CoronaPhysicalMtl?