You don't have authorisation to view this page. HTTP ERROR 403

I’ve recently upgraded from self-hosted CE to GitLab12.9.4-ee. So far, everything has gone fairly smoothly, except when working with the kubernetes integration. Sometimes when running a build, or a certain task runs in a CI process, the interface kicks me out and gives me a 403.

At the same time, the build usually fails too:

Checking out f6221c4b as feature/test...
 batch response: Authorization error: https://gitlab-ci-token:[MASKED]@gitlab.********/web-dev/********.git/info/lfs/objects/batch
 Check that you have proper access to the repository
 error: failed to fetch some objects from 'https://gitlab-ci-token:[MASKED]@gitlab.********/web-dev/********.git/info/lfs'

Once this happens, I cannot view the UI for a long time (hours, not minutes). The ONLY solution I have found to this is to SSH to the server gitlab is on, connect to redis:

/opt/gitlab/embedded/bin/redis-cli -s /var/opt/gitlab/redis/redis.socket

and to flush the entire instance:

redis /var/opt/gitlab/redis/redis.socket> flushdb

which of course logs me out in the process. I’ve checked all the logs and cannot find anything that seems relevant. Can anyone offer any tips to debug this issue?