Ambient room temperature was 22-23°C, in the 19" rack it get’s up to 28-29°C. Highest i ever measured inside the rack via network thermometer probe was 31°C.
SFP modules in use are Finisar FTLX8571D3BCL, which should be good up to 70°C. Right now both are running at 35 and 36°C.
In the end, my issues of instability might have come from the old firmware. Deployed both the fan mod and the upgrade to version 2.3 at the same time, so I’ll never know.