Just discovered the same.. 
Haven’t touched the filters, or the source but they look like below.
For me, it is also suddenly a language mix between swedish and english.
Got this issue for both Piwik and Data sheets as sources.
Saknas=Missing
For the source Piwik PRO I can see the fields, and they are green and correct, but as soon as I select one I got the red “Saknas” (Missing)
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