But logwatch does not understand the format of the nextcloud.log file. Logwatch comes with preset, what it calls, services (e.g. http to scan access logs of apache/nginx). I couldnt’t find one that could do the same for nextcloud.
logwatch and logcheck often come with some default settings for known webservices (apache, mysql, …). For nextcloud, you probably need to configure it yourself. I know with logcheck that you can specify with regexp the kind of lines that can be omitted (normal events that should not trigger alerts).