Windows – The filename, directory name or volume label syntax is incorrect while starting Task Manager or any program from System32 in Win10

Delete non-valid entries from the TaskCache. The TaskCache is normally maintained automatically, in this case some entries were not removed automatically when the task was deleted.

Search in registry and manually delete any keys that contain the non-valid scheduled task name that are found in:

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tasks

and

HKLM\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Schedule\TaskCache\Tree

This will allow reuse of the task name.

Other users with this scenario may also have to delete entries found in C:\Windows\System32\Tasks containing the task name, although in this case, you noticed these had already been deleted.

That was almost certainly caused by a corrupt AutoRun entry.

From the cmd /? help:

If /D was NOT specified on the command line, then when CMD.EXE starts, it looks for the following REG_SZ/REG_EXPAND_SZ registry variables, and if either or both are present, they are executed first.

HKEY_LOCAL_MACHINE\Software\Microsoft\Command Processor\AutoRun

   and/or

HKEY_CURRENT_USER\Software\Microsoft\Command Processor\AutoRun

Since deleting the profile fixed the problem, it must have been an entry in HKEY_CURRENT_USER. Quite how that entry got there is more of a puzzle, but probably just some sort of glitch.