When accessing Dynamics NAV 2013 or later through NAV Web Services, it is possible for the the error Input string was not in a correct format to occur.
This error can occur when an application attempts to access Web Services and the Services Language setting is configured for a language other than the recommended default of en-US:
Jet Reports does not recommend changing the "Services Language" away from "en-US" as this is the default fixed culture for the Microsoft SOAP Web Services.
Changing this back to "en-US" will still allow users to set their NAV RTC display language to another desired language and will not affect the overall functionality of the client's NAV instance.
Most applications, such as the Jet Excel add-in, are coded to work with Web Services in its default fixed culture (which is "en-US") so that all queries passed will work correctly and communications can be a constant.
Microsoft also notes that the default fixed culture for SOAP Web Services should be en-US to ensure that all third party applications work properly.
Another option, if you are still wary of performing this, would be to create a separate NAV instance specifically for the Jet add-in reporting with "en-US" as the "Services Language".
A separate NAV instance for Jet reporting is the recommended practice for performance as noted in the following knowledgebase article: Jet Reporting performance with NAV
With the "Services Language" set to en-US the client computer will still be able to set their local region and language settings to another desired language and use the syntax 01/01/15..31/12/15 in Excel.