TYPO3 Core

I just investigated it and problem is that there's no distinction between warning and error. The about_modules screen only knows one severity, which results in an alwaye ERROR-style message.

Wrong behavior nevertheless :( and as I already said, this is very confusing.

Can this be solved in an easy way or would it require a bigger change?

Should be no too big deal..

t3lib_BEfunc::displayWarningMessages() would need some changes to support warnings/errors.

A start could be to rename $warning to $error and pass both variables to the hook. Then adjust the code there and in sysext:reports to add the error/warning to the corresponding array.

Can you try to do this? Have no time...

This is not a bug. In this case the reports module is using the (back then) yellow warning box:

In 4.5, when you go to the reports module it detects the highest severity of statuses and stores that in sys_registry. Then when you go to the about module the reports moule takes over control of the warning box.
The value in sys_registry is read again and if higher than OK the message to check the reports is displayed.

The warning box has been red ever since it was changed from the old yellow to flash messages. AFAIK you can still see traces of the "yellow warning box" name in the code.