The /fill command, when given a block type that has a block entity such as mob_spawner, expects parameter 9 to be the NBT data, even if parameter 8 is "replace". As such, the following command:

/fill ~-5 ~-5 ~-5 ~5 ~5 ~5 minecraft:mob_spawner 0 replace minecraft:mob_spawner 0 { Abbreviation: "really long JSON string" }

throws this error:

Data tag parsing failed: Expected '{' but got 'm' at: m<--[HERE]