OPen PDF form failed; Xml-Analysis error: no element found (error code 3) line 1 column 1

Dear colleagues (especially the fellows from Adobe!),

we need an urgent help (or at least a suggestion).

Years ago we created a form using Adobe Experience Manager, now we try top open and edit it in Adobe Experience Manager -- Forms (Ver 6.4.0.20180228.1.932123), but receive an error

Xml-Analysis error: no element found (error code 3) line 1 column 1
and then a .tmp file path

What's wrong and how to get it edited?
To prevent possible questions:
- we tried different copies of this file - with the same effect
- the file gets easily opened in Adobe Reader and provides the expected form functinoalities

Thx in advance
Mike