Fails on start and sometimes does not accept connection

[rugk]

Long running on a server seems to be problematic.
Regularly I cannot upload any files as imgbi somehow closes the connection related to some headers. At least this is how I can interpret the nginx error messages (running as reverse proxy):

yyyy/mm/dd 12:59:18 [error] 12554#0: *1306 upstream prematurely closed connection while reading response header from upstream, request: "POST /SOMEDIR/api/upload HTTP/2.0", upstream: "http://127.0.0.1:SOMEPORT/api/upload"
yyyy/mm/dd 13:01:25 [error] 12554#0: *1306 connect() failed (111: Connection refused) while connecting to upstream, request: "POST /SOMEDIR/api/upload HTTP/2.0", upstream: "http://127.0.0.1:SOMEPORT/api/upload"

Or:

yyyy/mm/dd 18:39:57 [error] 1465#0: *174 recv() failed (104: Connection reset by peer) while reading response header from upstream, request: "POST /imgbi/api/upload HTTP/2.0", upstream: "http://127.0.0.1:SOMEPORT/api/upload"
yyyy/mm/dd 18:40:25 [error] 1465#0: *174 connect() failed (111: Connection refused) while connecting to upstream, request: "POST /SOMEDIR/api/upload HTTP/2.0", upstream: "http://127.0.0.1:SOMEPORT/api/upload"

After a restart of imgbi everything is working again.