I am currently uploading a huge amount of files (mainly documents and pictures) to a non-admin account via windows client version 2.3.0rc1 (build 4) and nextcloud log shows the following error every some minutes:
Error PHP Undefined offset: 3 at /path/to/nextcloud/apps/activity/lib/FilesHooks.php#620
So it has something to do with the activity app. As far as I can see there are also some of the files missing in the related users activity list.
But the upload works fine beside this, so count this more as report than as help request. If I should create an issue on github, I will do so.
I run Nextcloud 12 Beta 2
on a Raspberry Pi 2
with up to date Raspbian Jessie
on Linux kernel 4.9.25-v7+,