Hello @monstrator !
All your dataflows have the same problem? Interesting. Have you tried creating a new dataflow, something very simple, importing nothing but the key value and a single text field? This would just validate if dataflows stopped working completely or if only your previously built dataflows are at issue.
If the new dataflow with nothing but one field works, I would try this:
1. If you still have access to one of your previously imported datasets, try importing a single record from the old dataset (one that had a successful import) using your existing dataflow. If it works, the issue may be with the data.
2. If it doesn't work, try unmapping all your fields but one. If that works, add another field and refresh again until you identify the problematic field. If that doesn't work, try rebuilding your dataflows from scratch (or copy and paste from the advanced editor).
You will notice that the new datasets will be Standard V2. The differences between V1 and V2 are not clearly documented (to anyone: if I'm wrong and documentation exists, please reply below, I would love to read it). For sure, there have been back end changes lately. I've also had previously working dataflows "die" and I solved the issue by creating a new V2 dataflow, but some tweaking was required with dates and option sets (see here https://powerusers.microsoft.com/t5/Microsoft-Dataverse/Dataflow-Engine-V2-What-exactly-changed/m-p/818954)
Also, you said nothing changed except for the name of the environment. Did you consider if any new business rules were applied? I had another dataflow that failed because of a faulty business rules (see https://powerusers.microsoft.com/t5/Microsoft-Dataverse/Dataflow-error-Error-code-0x80040265-Message-String-was-not/m-p/823684)
Keep 'em flowin'
Charles
