SG330 unresponsive for a couple of minutes with postgres[4267] error

Question

Randomly during the day I might get this error: "postgres[4267]: [10-1] LOG: using stale statistics instead of current ones because stats collector is not responding" for a few minutes. 
 The my UTM becomes unresponsive for one minute or so and then i comes back to life. 
 The UTM is the SG330 with 9.355-1 firmware. 
 The CPU or memory load is low in general or during those errors. I cannot find any rational reason why this happens. Seems like a db bug to me. Thanks!

IliasTsoukalis · Accepted Answer

After a few months of usage and with the help of SebastianMeyer I can verify that disabling proxy cache resolves the issue. However, I don't know if a later version fixes the problem with proxy cache enabled since there is no need for cache mode to be enabled in our company. 
 Cheers!