When using Microsoft IIS as reverse proxy, some URLs may return an HTTP 400 error issued by IIS itself. Example of those URLs:

http://<proxy>/prweb/<...>/webwb/pzpega_ui_harnesscontext_1567413440!pzevalharnessjson_12337917940.js!clientlogbase_11632504602.js!pzpega_ui_lib_provider_1191955478!pega_yui_13189773860!desktopwrapper_13637517414!developerwrapperapi_12187829019.js!pzsyncappexplorer_11452689760.js!pzautomationscripts_1782369863!pzpega_portal_13220244661!pzpega_ui_jquery_11946158806!pega_ui_harness_13059245599!pzpega_ui_harness_actions_12368820038!pzpega_ui_crc32_1193200667!pzpega_ui_messaging_1861471868!pzpega_ui_jstree_1221119030!pzpega_ui_inspector_bundle_12577869144!!.js

http://<proxy>/prweb/<...>/webwb/pzpega_ui_decimal_13948517401.js!pzpega_ui_formatnumber_13702883570.js!pzpega_ui_markdown_it_13093754858.js!pzxss_1331852171.js!pzpega_ui_advancedtext_11383516646.js!pzpega_ui_template_displaytext_11649458985.js!pzpega_ui_calendar_util_11790586412.js!pzpega_ui_calendar_dbp_14237290422.js!pzpega_ui_calendar_1147015783.js!pzpega_ui_formatdatetime_12653965207.js!pzpega_moment_with_locales_171269180.js!pzpega_moment_timezone_with_data_14074664356.js!!.js

All HTTP requests in IIS obey to http.sys restrictions, see Microsoft Support - Http.sys registry settings for Windows for a list of those restrictions.
In particular, setting UrlSegmentMaxLength limits the length of strings between slashes in the URL to 260 characters by default. Above URLs are more than 500 characters long. Increasing this setting to 1000 will solve the issue (to be tuned).

As per Microsoft Developer - Response 400 (Bad Request) on Long Url, simply restarting IIS may not suffice for the change to be effective, a server reboot is required.

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Your friendly neighborhood Technical Architect

***Edited by Moderator Marissa to update Content Type from Discussion to Question***