Around the 7th May 2026 I upgrade my Matomo installation from the previous version to v5.10.1 and got a fatal error. I am now back to trying to resolve the issue.
Note my Motomo/Piwik installation is on a shared hosting platform. Yes it was originally installed when Piwik. I do not have have permissions to run Motomo CLI commands.
The initial fatal error is:
A fatal error occurred
Please contact the system administrator, or login to Matomo to learn more.
If you are Super User, but cannot login because of this error, you can still troubleshoot further. Follow these steps:
open the config/config.ini.php file and look for the salt value under [General].
edit this current URL you are viewing and add the following text (replacing salt_value_from_config by the salt value from the config file):
I followed the suggestion and then get the following fatal error:
A fatal error occurred
The following error just broke Matomo (v5.10.1):
variable @theme-color-border-alternative is undefined in file anonymous-file-0.less in anonymous-file-0.less
in /home/ffenestr/public_html/piwik/core/AssetManager/UIAssetMerger/StylesheetUIAssetMerger.php line 54
I have tried the trouble shooting and further trouble shooting suggestions with no joy.
I would really appreciate some guidance on how to to resolve the fatal errors.
The @theme-color-border-alternative is undefined error looks more like a problem with Matomo’s compiled UI assets/theme files than with the database itself.
Since you’re on shared hosting and can’t use the Matomo CLI, I’d first try clearing Matomo’s generated cache/assets manually. If you have access to the Matomo installation files, check the tmp directory and remove the generated cache contents (while leaving the directory itself in place), then reload Matomo so it can rebuild the assets.
I’d also check whether you have any custom themes or third-party plugins installed. An older theme/plugin that isn’t compatible with Matomo 5.10.1 could potentially be referencing a LESS variable that no longer exists.
If possible, temporarily disable third-party plugins/themes by renaming their directories and then test again. Since this is an upgrade from an older Piwik installation, compatibility with older extensions is definitely worth checking.
Before deleting or changing anything, make sure you have a backup of both the database and the Matomo files.
If clearing the generated assets and disabling third-party extensions doesn’t help, I’d post the exact Matomo version you upgraded from and your PHP version as well. Those details may help narrow down whether this is an upgrade compatibility issue.
The @theme-color-border-alternative error looks like the stylesheet merger is loading a theme variable that isn’t available after the upgrade. Since this is running on shared hosting without CLI access, I’d check whether the upgrade left any old cached assets or theme files behind. Clearing Matomo’s generated asset/cache files and then rebuilding the UI assets, if possible through the web interface, might help identify whether the problem is caused by stale files or the theme itself.
Hi @osprey the theme-color-border-alternative error sounds like it could be related to a theme or asset issue after the upgrade. Since you’re on shared hosting without CLI access checking whether the active theme or custom CSS is causing the LESS variable error might be a useful next step.