Data Merge - Can't preview without errors
The apparently long-running Data Merge previewing error!
Occasionally previewing a data merge will permanently ruin exports. The only solution is to create a new document. Happened to me on almost every project I tried - wasted hours and hours.
See this thread and video for more information:
https://forums.adobe.com/message/10337647#10337647
(Also colons in csv titles makes file unusable??)

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Paul Nylander commented
I've found this error occurs if I have other page elements, such as a footer in the master page.
And, yes, as of 2022's Id 17.4, this bug is still present.
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Anonymous commented
Same here, data merge would not work on ANY existing document, but worked flawlessly on a new document w/o formatting.
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Gemma commented
This does waste hours, previewed early on and spent a ridiculous amount of time assuming my files weren't set up correctly when it was InDesign the whole time.