BIG-IP : create data-group fails with format error

f5 BIG-IP LTM VE v11.4.0 on ESXi

I am attempting to create a new data-group :

Type = "(External File)"
File Contents = "String"
Key/Value Pair Separator = ":="

My external data-file 'test.txt' :

"test-1" := "test-1",
"test-2" := "test-2",

What is strange is that I've previously successfully loaded data in exactly this format.

However, now I receive this error :

0107169a:3: The data group external file (/config/filestore/.stage_d/107479_d/Common_d/data_group_d/:Common:test_57194_1) has an invalid format, line: 2.

I've tried removing the final comma on line 2 :

"test-1" := "test-1",
"test-2" := "test-2"

but receive the same error.

With a single row :

"test-1" := "test-1",

the data loads with no error.