The proxy have a policy for ICAP Request mode to send the object to a DLP server.
User try to upload a file but failed to do so and the developer tool on the browser is showing a 503 response from the proxy.
When bypassing SSL interception, it works fine.
The issue is due to the DLP server is blocking or dropping the uploaded file.
The proxy HTTP debug shows the following error:
4034.013 HTTP Explicit HTTP INFO HTTP CW 193BB730A40 PEXID: 25 (ICAP Error)
4034.013 HTTP Explicit HTTP INFO HTTP CW 193BB730A40 exception id 25
4034.013 HTTP Explicit HTTP INFO HTTP CW 193BB730A40 Terminating connection with response code 503
4033.470 HTTP Explicit HTTP INFO HTTP CW 193BB730A40 request from: 10.10.10.11, URL: https://www.example.com
The packet capture will show the proxy sent a TCP SYN packet but didn't get a response or the initial TCP handshake with the DLP server was successful but there was no ICAP respond.
This is more of an issue on the network side or the DLP server.
Create a rule in the proxy to bypass DLP scanning and the user able to upload the file without any issue.