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.

It might help. It surely won't hurt, so please grab it.
Also, could you try running the latest beta version and see whether it makes any difference?

Should be fixed in v4.5.6.9 (beta).

The feature is available as an advanced option (show_desktop_on_hover) in v4.5.6.9 beta.