This has been happening a couple of times already, just redoing it would fix it but it's annoying:
11:38:26 -'<img src="05207d8083caa922b443937de1b6b14a4d3a7335" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -0.338ex; width:4.572ex; height:2.176ex;" alt="{\displaystyle \sin{x}}"/>'
11:38:26 +'<strong class="error texerror">Failed to parse (MathML with SVG or PNG fallback (recommended for modern browsers and accessibility tools): Invalid response ("Math extension cannot connect to Restbase.") from server "https://wikimedia.org/api/rest_v1/": {\displaystyle \sin{x}}</strong>\n
11:38:26 +'
11:38:26
11:38:26 /workspace/src/extensions/Math/tests/phpunit/MathCoverageTest.php:40
11:38:26 /workspace/src/tests/phpunit/MediaWikiIntegrationTestCase.php:418
11:38:26 /workspace/src/maintenance/doMaintenance.php:99or
13:16:45 1) MathCoverageTest::testCoverage with data set #30 ('dt, \operatorname{d}\!t, \par...\psi\!', array(), '<img src="68958802ac6c403e4a1...\!}"/>')
13:16:45 === Logs generated by test case
13:16:45 [objectcache] [debug] MainWANObjectCache using store {class} {"class":"EmptyBagOStuff"}
13:16:45 [Math] [debug] Start rendering ${\displaystyle dt, \operatorname{d}\!t, \partial t, \nabla\psi\!}$ in mode png []
13:16:45 [Math] [info] Tex check failed {"post":{"type":"tex","q":"{\\displaystyle dt, \\operatorname{d}\\!t, \\partial t, \\nabla\\psi\\!}"},"error":"(curl error: 28) Timeout was reached","urlparams":"https:\/\/wikimedia.org\/api\/rest_v1\/media\/math\/check\/tex"}or
13:13:03 1) MathCoverageTest::testCoverage with data set #4 ('\sqrt{\pi}', array('square root of pi'), '<img src="7ae18ec124928c74818...i}}"/>')
13:13:03 === Logs generated by test case
13:13:03 [objectcache] [debug] MainWANObjectCache using store {class} {"class":"EmptyBagOStuff"}
13:13:03 [Math] [debug] Start rendering ${\displaystyle \sqrt{\pi}}$ in mode png []
13:13:03 [Math] [error] Restbase math server problem {"urlparams":"https:\/\/wikimedia.org\/api\/rest_v1\/media\/math\/render\/mml\/7ae18ec124928c74818b516e6350ca9610966c6e","response":{"code":0,"body":""},"math_type":"mml","tex":"{\\displaystyle \\sqrt{\\pi}}"}or
13:16:45 3) MathCoverageTest::testCoverage with data set #86 ('\and \land \wedge, \curlywedg...dge \!', array(), '<img src="bdb145fadf20142e514...\!}"/>')
13:16:45 === Logs generated by test case
13:16:45 [objectcache] [debug] MainWANObjectCache using store {class} {"class":"EmptyBagOStuff"}
13:16:45 [Math] [debug] Start rendering ${\displaystyle \and \land \wedge, \curlywedge, \bigwedge \!}$ in mode png []
13:16:45 [Math] [error] Restbase math server problem {"urlparams":"https:\/\/wikimedia.org\/api\/rest_v1\/media\/math\/render\/mml\/bdb145fadf20142e514b40731ed8c431dc53d855","response":{"code":0,"body":""},"math_type":"mml","tex":"{\\displaystyle \\and \\land \\wedge, \\curlywedge, \\bigwedge \\!}"}
13:16:45 [MessageCache] [debug] MessageCache using store {class} {"class":"HashBagOStuff"}
13:16:45 [Math] [error] Cannot get mml. Server problem. [{},{}]
13:16:45 ===
13:16:45 Failed to render \and \land \wedge, \curlywedge, \bigwedge \!
13:16:45 Failed asserting that two strings are equal.
13:16:45 --- Expected
13:16:45 +++ Actual
13:16:45 @@ @@
13:16:45 -'<img src="bdb145fadf20142e514b40731ed8c431dc53d855" class="mwe-math-fallback-image-inline" aria-hidden="true" style="vertical-align: -1.338ex; margin-right: -0.257ex; width:11.969ex; height:3.843ex;" alt="{\displaystyle \and \land \wedge, \curlywedge, \bigwedge \!}"/>'
13:16:45 +'<strong class="error texerror">Failed to parse (Conversion error. Server ("https://wikimedia.org/api/rest_") reported: "Cannot get mml. Server problem."): {\displaystyle \and \land \wedge, \curlywedge, \bigwedge \!}</strong>\n
13:16:45 +'In summary we see different random errors while interacting with restbase from the CI. This is that either the check or the mml request fail. On top it seems that different HTTP implementation are use curl Guzzle. One might suspect that the reporting of HTTP-Status Code 0 points to a consequtive error in the MW-Guzzle implemenation (T232866).