We also ran into the same problem. Unlike @evilmav we use a Kasli-SoC as master and a Kasli v2 as satellite. The DRTIO link is also setup with transceivers recommended in the Sinara wiki on github. We use the most recent version from artiq-zynq ARTIQ v8.8954+4235309 (for building the gateware for both crates and the software on the host).
We used the artiq_sinara_tester and it seems like the error messages (see evilmav) only appear when a TTL device is involved. When the DIO cards or the Urukuls (because of the RF switches, which appear as TTL devices in the device_db) test routines are executed we see these errors. But if we only execute the test routine for the Fastinos there are no errors.
We also switched the roles of the crates, so the Kasli v2 is now the master and the Kasli SoC is the satellite. Then we don't see the communication errors at all when we run the sinara tester.