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This one came down to the formatting of the request. Here is the correct way to format the data:
var data = JSON.stringify({
TableName: 'prospect-materials',
ExpressionAttributeNames: {'#lang':'LANGUAGE'},
ExpressionAttributeValues:{
':m':{S:'Acet'},
':c':{S:'CH'},
':l':{S:'DE'}},
FilterExpression: 'begins_with(MAT_DESC, :m) and COUNTRY = :c and #lang = :l'
})
Source:stackexchange.com