What is odd for me is that the entire syringe assembly (syringes and block included) and all the tubing is used for all fluid movements, yet you only have trouble when replacing media. Why would the syringe block behave differently for just that function? But the leaking elbow fitting you show happens to be the one on the replacement media reservoir. Do you think that the leak could be dropping the level enough so that the needles just enter the surface and pull some air?
We are about to resume shipments of the filter module and will be releasing an upgrade kit for the 850 base unit with this release. The kit replaces the reservoir block and inverts the syringe pump assembly. If it is acceptable to wait another week or so for the upgrade kit to finish the ECO process I would like to send one of these kits for the repair. The kit will not replace the manifold block but in the meantime you could verify if this is indeed the issue and let us know if an additional part is required.
Let me know if that sounds like an acceptable plan.