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I'm able to reproduce this issue my side.

if the graphql query used variables in parameters then always expected to send the value of the query variables to be {}. if we pass query variables value to be null then this issue occurs.

Expected Request:

{"query":"query myquery($uri: String = "tcm:5-394-64") \n{ \n
page (cmUri:$uri) {\n id,\n \titemId,\n
\tlastPublishDate,\n title,\n \turl,\n \ttaxonomies{\n
id,key,title\n }\n }\n }\n","variables":{},"operationName":"myquery"}

I hope it helps.