Sorry, my fault. Because of the error message with GitHub I have set the ugm path local to a JSON file (like described in this old post: Error in UpgradeMODX widget ("403 API limit exceeded") - #11 by halftrainedharry) and there was a wrong path (typo) in it.
I have a new site, built with 3.0.3 from scatch. UpgradeMODX extra is installed and shows this red error message in my dashboard:
Call to undefined method GuzzleHttp\Exception\ConnectException::hasResponse() Errors: * #0 /mnt/…/htdocs/www/core/components/upgrademodx/model/upgrademodx/upgrademodx.class.php(680): UpgradeMODX->parseException() * #1 /mnt/…/htdocs/www/core/components/upgrademodx/model/upgrademodx/upgrademodx.class.php(802): UpgradeMODX->getRawVersions() * #2 /mnt/…/htdocs/www/core/cache/includes/elements/modx/revolution/modsnippet/44.include.cache.php(99): UpgradeMODX->upgradeAvailable() * #3 /mnt/…/htdocs/www/core/src/Revolution/modScript.php(88): include('...') * #4 /mnt/…/htdocs/www/core/src/Revolution/modDashboardSnippetWidget.php(33): MODX\Revolution\modScript->process()
and so on.
No error messages in the MODX error report.
It’s the only one site with this error. 16 other sites on 3.0.3 with UpgradeMODX don’t have this message. One site is installed at the same provider: also without error messages.