Failed to download turris-ipsets.gz.sign

I’m going through error log and this entry is repeating:

2016-11-30T07:02:09+01:00 omnia user err turris-firewall-rules[]: (v61) Failed to download

The error seems to come from /usr/share/firewall/turris-download script. I tried to reconstruct and it seems that the error is generated when executing

curl -fs --cacert /etc/ssl/startcom.pem --crlfile "/etc/ssl/crl.pem" "" -o "/tmp/fw-rules/turris-ipsets.gz.sign"

This works fine when executing from the router and also when I checked the /tmp/fw-rules folder the file turris-ipsets.gz.sign was there (already before calling the curl command manually).

The script seems to run every hour - it failed 5 times yesterday and already twice today. Clearly it doesn’t fail every time and I doubt there are internet outages with my ISP so often.

So I’m wondering why are the errors generated? Could it be that the file is not available at at some times (e.g. during the file update)?