We apologize for the inconvenience caused, as per the description above, you are not able to send or collect signatures using Acrobat 11, Is that correct?
Please check for any pending updates of Acrobat from help>check for updates, reboot the machine after installing the updates, and see if this brings any difference.
Are you using Send for Signature or Fill and Sign feature? Is it possible to share a screenshot of the error you are getting? To share a screenshot, please refer to https://forums.adobe.com/docs/DOC-7043#jive_content_id_How_do_I_attach_a_screenshot
Note: Acrobat 11 is already declared as an End of support product, End of Support means that Adobe no longer provides technical support or distributes runtimes. This policy affects product and security updates for all derivatives of a product or product version (localized versions, minor upgrades, operating systems, dot and double-dot releases, and connector products).
For detailed information, refer to End of support for Adobe Acrobat XI and Reader XI
Let us know how it goes and share your findings.