Possible scenarios are when there is a Cluster environment where link sent by one node works and the other node doesn't. This can happen in standalone environment as well.
How to fix issues related to these link/URL not working ?
1 ) ENTITY_LINK_HOST : This parameter enables you to set the mail server host name used in the link to an entity when ALM mails an entity.
2 ) ENTITY_LINK_PORT : This parameter enables you to set the mail server port number used in the link to an entity when ALM mails an entity.
NOTE : By default, ALM uses the default port number and host name specified during installation. These two parameters are optional and are available for QC 10 and above versions.
Setting the above parameters to correct values helps in fixing some of the issues faced with accessing the links/URL.