Using the @e selector in place of "<selector>" in the following:

/scoreboard players operation <targetName> <targetObjective> <operation> <selector> <objective>

Will issue the "entity UUID provided is in an invalid format" error, so long as the selector is not targeting a player:

/scoreboard players operation @e OBJECTIVE += @e[type=!Player] OBJECTIVE

Essentially meaning you cannot apply scoreboard operations against entities using another entity's score, which worked in previous snapshots. If replacing "<targetName>" with a selector capable of only targeting a player will work as expected:

/scoreboard players operation @p OBJECTIVE += @e[type=!Player] OBJECTIVE