I am Adobe customer, and Adobe have left me alone.

Acrobat XI is End of Life. Adobe discontinued XI in 2017. 

https://helpx.adobe.com/acrobat/kb/end-of-support-acrobat-xi-reader-xi.html

For security & compatibility reasons, you should not use obsolete apps anymore, especially when newer & better options are available. See below.

FREE Acrobat Reader -- view, comment, print & sign PDF.
https://www.adobe.com/acrobat/pdf-reader.html

FREE Online PDF Editor

https://www.adobe.com/acrobat/online/pdf-editor.html

Paid Acrobat Pro -- the complete solution for creating and saving PDF.
https://www.adobe.com/acrobat/acrobat-pro.html

Hope that helps.

adobe supports info isn't accurate.  @Ben107584 M 's info is correct.

in particual, the vendor is almost certainly not adobe and you were sold a volume license that was illegally sold to individuals.  ie, the vendor is the culprit.

The error you received does not relate to activation servers being down. If the license was revoked it is possible that the license you were sold was not valid from the reseller and had been misused, therefore it was revoked. Details on the error can be found here: https://helpx.adobe.com/x-productkb/global/invalid-revoked-serial-numbers.html .

The license you mention is not a single user perpetual license though. The license is a volume license purchased by a business to distribute amongst an entire organization. This means that all users of your organization will be impacted. If this is your volume license you can reach out to your license administrator to look into this further to investigate if this is a valid license. If you are unsure what this means or had bought this individually for yourself, this is likely the issue that the version sold you to was not a perpetual individual license, but a volume license intended for enterprise organizations and the license could have been missold to you in which case you can try to go to the vendor for a remedy.