A quick “info” post in case this could be of use to others.
For basic context, my instance is hosted on a classic LAMB provider.
During my most recent update rounds (usually done via the web updater), I’ve had to resort not only to occ, but also tinker with “/lib/private/Http/Client/Client.php” for the update to succeed.
Via the Web updater, the update partially proceeds with green ticks up to the “DB update” line, then fails on the next step (red X followed by an empty line).
Resorting to occ upgrade, I was faced with the following errors:
Setting log level to debug
Updating database schema
Updated database
An unhandled exception has been thrown:
Error: Undefined constant "CURL_HTTP_VERSION_2TLS" in /domain.xxx/lib/private/Http/Client/Client.php:52
Stack trace:
#0 /domain.xxx/lib/private/Http/Client/Client.php(214): OC\Http\Client\Client->buildRequestOptions(Array)
and (upon resolution of the above)
Setting log level to debug
Updating database schema
Updated database
An unhandled exception has been thrown:
Error: Undefined constant "OC\Http\Client\CURL_VERSION_BROTLI" in /domain.xxx/lib/private/Http/Client/Client.php:91
Stack trace:
#0 /domain.xxx/lib/private/Http/Client/Client.php(214): OC\Http\Client\Client->buildRequestOptions(Array)
Upon research and tinkering, it appears this might be due to those constants possibly not available in PHP 8.3 (running 8.3.32 in my case), but versions 8.4 or 8.5 aren’t available yet on my hosting.
I’ve tried a variety of suggestions relating to “CURL_HTTP_VERSION_2TLS” and “CURL_VERSION_BROTLI” errors, but with no positive outcome, so here’s what I tried that worked in my case.
Edit the following:
$defaults['curl'][\CURLOPT_HTTP_VERSION] = \CURLOPT_HTTP_VERSION; (line 52 in my case)
to
$defaults['curl'][\CURLOPT_HTTP_VERSION] = 84;
and
if (((curl_version()['features'] ?? 0) & CURL_VERSION_BROTLI) === CURL_VERSION_BROTLI) { (line 91 for me)
to
if (((curl_version()['features'] ?? 0)) === 8388608) {
Those will also have to be left as is for the NC instance to work (therefore I ignore the integrity warning for that file).
After those edits, I was able to update/upgrade successfully. I find it quite strange however that NC code would resort to constants that only exist in PHP 8.4+, as the docs says PHP versions from 8.2 to 8.5 are OK. Maybe this is a PHP settings anomaly at my provider ? I considered filing a bug, but NC’s github says “No server is currently available to service your request” at the moment 
Doing some additional search as part of this, the above might be a mix of PHP and Curl version “trouble”.
Here’s what led me to doubt constant definition availability for CURLOPT_HTTP_VERSION and PHP 8.3 CURLOPT_HTTP_VERSION Constant • PHP.Watch
For CURL_VERSION_BROTLI , the Curl version might be the main issue in my case, as my provider shows curl 7.29.0 but the "constant availability is in the same state CURL_VERSION_BROTLI Constant • PHP.Watch
I’ll have a word with my provider though, because that curl 7.29 seems to be 13+ years old curl - Release Table