This basically the same bug as #1894907, but there I decided to disable docs rebuilding, after checking that none of the patches were against the docs source.
Furthermore, we should probably fix these lintian issues:
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/developer.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/download.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/genindex.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/getsasl.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/index.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/operations.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/packager.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/search.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/setup.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2-doc: privacy-breach-uses-embedded-file usr/share/doc/cyrus-sasl2-doc/support.html You may use the libjs-mathjax package. (https://cdn.mathjax.org/mathjax/latest/mathjax.js)
E: cyrus-sasl2 source: source-is-missing doc/html/_static/jquery.js line length is 32014 characters (>512)
E: cyrus-sasl2 source: source-is-missing doc/html/_static/js/modernizr.min.js
E: cyrus-sasl2 source: source-is-missing doc/html/_static/underscore.js line length is 519 characters (>512)
E: cyrus-sasl2 source: source-is-missing docsrc/exts/themes/cyrus/static/js/modernizr.min.js
E: cyrus-sasl2 source: source-is-missing docsrc/exts/themes/sphinx_rtd_theme/static/js/modernizr.min.js
A fix was released in the latest impish sync (2.1.27+dfsg-2.1), where the sphinx builds were re-enabled.