I should have posted this on the G3 Forums, but currently I can't access my account there because of a technical problem. This bug report is meant to both inform the current maintainer of the mod, if there is one, and to provide workarounds to affected players.
When dismissed by the party, Amber normally goes to the Five Flagons Inn (area AR0509) where she can be recruited again. However, if the player owns the Bard Stronghold, area AR0509 is no longer accessible and it is replaced by area AR0522, so Amber will be nowhere to be found and it will not be possible to recruit her back.
Several other mods, like Keto or the The Gibberlings Three Anniversary Mod, have code to handle a similar situation (by checking if the player owns the Bard Stronghold and eventually moving creatures to the right area) and could provide to the maintainer of Amber an example of how to fix this bug.
Affected players can use one of the following workaround (until the bug is fixed):
1) use the CLUA Console to move the party to area AR0509, recruit Amber and then leave the Five Flagons Inn by the door for the Bridge District as usual
2) use NearInfinity to edit a savegame and move Amber to another area (the picture under the spoiler tag shows how to do it)