Is "an archives" correct? If so, is it an exception or are there others like it?

I know of at least one instance where the repository of archival material is officially designated as "the Archives", an institution.

See also,
Glossary of Recordkeeping Terms [emphasis mine]

Archives
Those records that are appraised as having continuing value. AS 4390 Part 1 Clause 4.5
Traditionally the term has been used to describe records no longer required for current use which have been selected for permanent preservation. Also referred to as permanent records.
The place (building/room/storage area) where archival material is kept.
An organisation (or part of an organisation) responsible for appraising, acquiring, preserving and making available archival material.

Access
The granting of permission to:
• use the reference facilities of an archives
• examine and study individual archives and or collections held by archives

theFreeDictionary:

Noun 1 archives - collection of records especially about an institution
compendium, collection - a publication containing a variety of works

Another very important example of a word that looks plural but is treated as a singular is "United States of America". In all languages except English it is treated as a plural, les États Unies de l'Amerique, Соединенные Штаты Америки, etc. The phrase was treated as a plural in English prior to the Civil War. That is back then you would refer to "these United States of America". That War established that even though individual states had voluntarily joined they did not have the authority to unilaterally terminate their membership in the Union. It was the policy of the victors to emphasize the unity of the republic, exemplified by Lincoln's repeated use of "nation" in the Gettysburg Address, a word that literally indicates a people with a common ancestry as it continues to be treated in most other languages. This unity is also drilled into Americans by the daily repetition of "one nation" in the Pledge of Allegiance.

By the way the phrase “the Government of the People, by the People, and for the People.” in the Gettysburg Address was a quote from the forword to the first English translation of the Bible by John Wycliffe in 1384.