Lot of packets dropped - Why?


I finally decided to monitor my Nextcloud installation with Netdata.
Now, I see that I have 2 issues regarding dropped packets, as you can see below:

I don’t know how to trouble shoot this.
I read this topic to find the root cause but the result won’t help me that much (I won’t know what to do with the kernel function in cause).

Some information about my installation:
Nextcloud version: 17.0.3
OS version: Ubuntu 18.04.2 LTS
OS Codename: bionic
This is the Nextcloud VM – T&M Hansson.

Thank you for your help!