Only googlebots having error: Could not load file or assembly 'AjaxControlToolkit

Only googlebots having error: Could not load file or assembly
'AjaxControlToolkit

I had updated the AjaxControlToolkit from 3.5.51116.0 to 3.5.7.123 to get
ride of this error: System.NullReferenceException: Object reference not
set to an instance of an object.
The above error is gone but according the site error log table googlebots
are now having the following error randomly.
Could not load file or assembly 'AjaxControlToolkit, Version=3.5.51116.0,
Culture=neutral, PublicKeyToken=28f01b0e84b6d53e' or one of its
dependencies. The located assembly's manifest definition does not match
the assembly reference.
Please note that site visitors are not having this error at all only
googlebots having this error randomly and I don't know why?