We solved this with @bozana (thanks!)
The problem was that our site has five journals using subdomains and these subdomains are defined in config.inc.php with base_url settings. The rest of the journals are using the default url format.
However, while debugging this we noticed that only the journals with base_url’s defined were getting hits in the metrics table. The way I solved this was that I added base_url settings for all journals to the config.inc.php, so also for those that use the default http://domain.fi/journalname/
Although this is probably solved now, I do think that the way OJS handles the base_url settings should be changed someway. I recall a similar issue: OJS3 rewriting subdomains for journals, problem with admin panel journal drop down - #8 by asmecher
Edit: just to confirm that the usage statistics were now processed with 0 errors.