I had a few days of problems with Nextcloud, OpenVPN and even SSH applications crashing out complaining about HMAC/MAC errors. These are the message digests which are sent between the two hosts that provide verification of packet integrity.

To help Google lead you here, the exact error for OpenVPN on the client side was:

Authenticate/Decrypt packet error: packet HMAC authentication failed

After spending hours trying to figure out whether my TLS libraries were bust and asking Scaleway themselves if they were having networking troubles, I decided to power down the server and 'Archive' it to use Scaleway's terminology.

Brought it back up about two hours later... Everything was working just fine.

The only thing I can think happened is that whatever network segment or switch port I was on was having some sort of trouble. By pushing my server to their archive system and then bringing it back I must have re-triggered the network deployment and something must have sorted itself out.

Hope this helps anyone with similar troubles.