After update QTS 5.2.5.3145, my TS-873U-RP having same web login issue
(Proxy Error The proxy server received an invalid response from an upstream server. The proxy server could not handle the request Reason: Error reading from remote server)
Yesterday rollback firmware to 5.2.4.3092 build 20250403 for fixed
Today checked, 5.2.4.3092 build 20250403 seem removed on official web
( https://download.qnap.com/Storage/TS-X32U/TS-X32U_20250403-5.2.4.3092.zip )
TS-832PXU-RP on QTS 5.2.5.3145 has no issue