I’m not totally sure, but it could be timing out from either dormancy on the server or perhaps the client side connection. I had a slight issue with this myself that I resolved by specifying a longer timeout in the ini. for the client. I hadn’t had to tinker with anything in that regard on the server itself. Yet anyway…
My issue was spurred from unbeknownst packet loss on my local network causing the connection to timeout before authentication. Running some packet loss tests revealed this and I later identified an external SSD drive causing interference on my network. 
I hadn’t noticed until actually finally uploading my servers and connecting to and from them on my physical remote servers in another state, which revealed this issue is way more sensitive when travelling a distance.
The specified timeout was great in mitigating “lag/packet loss”, but only masking my issue. I lost some hair figuring that out and that may not be your issue, but I’d look into making sure your ISP connections are solid before running your code amok like I did lol.