Has anyone encountered log entries such as this after alerts have stopped flowing?
Some older versions of Alert Manager did not explicitly include the locale specific resource file alertmanagercore.en-US.resx. However, this should not keep alerts from being sent as it should refer back to alertmanagercore.resx when it cannot find the local specific file. But the error message will be logged if it is missing.
Later builds include the locale specific US english version of the resource file(s), specifically to avoid this error.