Addition/correction to my note from above.
The respawn menu is working correctly, with Side/Namespace target parameters. The problem was, that BIS_fnc_addRespawnPosition expects the position in an [x, y, z] format, providing only [x, y] coordinates will result in the error described above. The client menu respawn dialog will not show the correct markers then and the respawn not work.
Maybe this can be added in the description for the function.
Summary:
_CreatedRespawnPositionArray = [WEST, [1, 1], "Respawn menu string"] call BIS_fnc_addRespawnPosition; -> will NOT work.
_CreatedRespawnPositionArray = [WEST, [1, 1, 0], "Respawn menu string"] call BIS_fnc_addRespawnPosition; -> works as intended.