The Commerce Engine might become unavailable intermittently due to concurrent HTTP requests blocked on sync-over-async calls. The issue can result in request timeouts and the 502 status code.
To resolve the issue, download and install the hotfix:
For Sitecore Commerce 8.2.1 Update-2: SC Hotfix 38561-1 Commerce.Engine 1.2.237.zip
Be aware that the hotfix was built for the specific Sitecore Commerce version and must not be installed on other Sitecore Commerce versions or in combination with other hotfixes. In case any other hotfixes have already been installed on a certain Sitecore Commerce instance, send a request for a compatibility check to Sitecore Support.
Note that the ZIP file contents must be extracted to locate installation instructions and related files inside. The hotfix must be installed on a CM instance and then synced with other instances using standard development practices.