It may happen that the User Interface of Matomo fails to load and display an error message which can mention Unable to write in the cache directory or The directory tmp/cache/* is not writable. This indicates there is a small issue around folders and/or file permissions within the Matomo directory. The full error message could be A fatal error occurred. The following error just broke Matomo (v3.10.0): An exception has been thrown during the rendering of a template "Unable to write in the cache directory"

These issues can be addressed by setting the correct permissions that will let processes read and write the tmp/ folder.

General solution: if your web server username is www-data then you can have your webserver own the files in this directory (replace www-data by the web server username, sometimes called apache):

sudo chown -R www-data:www-data /var/www/html/matomo/tmp/*

If your server is running SELinux or CentOS, you may be able to fix the issue with the following commands:

chcon -t httpd_sys_content_rw_t /var/www/matomo/tmp
chcon -R -t httpd_sys_content_rw_t /var/www/matomo/tmp/tracker/
chcon -R -t httpd_sys_content_rw_t /var/www/matomo/tmp/cache
systemctl reload httpd.service

If this error keeps occurring, the issue may be that the scheduled task (crontab) for the core:archive console command is running and writing these files as a different user. To solve the issue you want to modify your crontab so that it runs the core:archive command as the web server user, for example running as www-data or apache. See the the user guide for more details.

It has also sometimes helped to upgrade PHP to use a recent PHP release.

If you still experience this issue after following the steps above, please get in touch with us at matomo.org/contact and we will help troubleshoot further.