Unable to upload non-tiny files, streaming music is choppy

When I try to upload any file above a certain size (as low as 50kb), I will get results like ERR_SSL_BAD_RECORD_MAC_ALERT in Chrome, SSL_ERROR_BAD_MAC_ALERT in Firefox, and a message about 'unexpected dropped connection' in Safari. The easiest place to test this is at https://www.csm-testcenter.org/test?do=show&subdo=common&test=file_upload I cannot tell whether this is hardware or software (OS level) related, but I do know that it's only affecting my MacBook Pro (14-inch, Nov 2023, M3) with MacOS Sonoma 14.5 (23F79), and not other devices. It happens when using any Wi-Fi network, and also when connected via an adaptor to a wired network. I have cable broadband which gets speeds around 900mbps (up and down). Streaming music (e.g. from Amazon Music) is choppy and irritating. Any advice or suggestions will be appreciated, as this is a daily frustration. Any other way I can perhaps narrow down what's going on? (some logging somewhere?)