having the tab arrive in a predictable position (say, near where it's dropped, or always at the end of the last row) after right-dragging the tab would be more useful

That's the intent, and it works pretty well for a single row, but for multiple rows it gets more complicated, especially when there are groups which span through multiple rows. How about dragging it to the upper left corner? Then it will end up being the first item.

Note: I understand about the Explorer thing where navigating folders causes the Explorer task/tab to jump out of chosen group and back to default Explorer.exe group.

Check out v4.5.8.2 beta ;)

With the Drag within/between groups using right mouse button option, dragging a button out of the taskbar will create a new group. Seems like that's what you're looking for.

I believe that the issue might have been the server load. In the last few hours, the server responsiveness was not ideal, probably due to all the users which try to update to the newly released version.
Anyway, I'm glad that you got it sorted.

Could you please provide the exact error message, and/or a screenshot? Thanks.

Fixed in v4.5.8.

Great, thanks. But before releasing it as v4.5.8, I'd like to get more feedback.
So if somebody reads it and encountered the same issue, please confirm that the version above works.

Please try the following version, and let me know whether it works:
https://www.dropbox.com/s/j4c55cgk0m7nvph/7tt_setup_beta_v4.5.7.1.exe?dl=0
Thanks.

Should be fixed in v4.5.7.

v4.5.7: Added a new advanced option, show_desktop_on_hover, which makes the taskbar to show desktop when a file is dragged over it (similar to Windows XP).

Done in v4.5.7: If the file "7+ Taskbar Tweaker.ico" exists, it's used as a tray icon.