at decoded URL1:711:577 at $.cookie URL1:712:260 at get URL1:822:1424 at sessionId URL1:867:276 at ctaShown URL1:11:976 at fire URL1:155:209 at fireWith URL1:156:402 at fire URL1:806:482 at CXContributions.prototype.render URL1:5:2077 at CXContributions.prototype.init URL1:5:1458 at CXContributions URL1:5:1403 at URL1:6:821 at mightThrow URL1:158:149 at resolve/</process< URL1:158:808 URL1: https://sq.wikipedia.org/w/load.php?lang=fr&modules=ext.centralNotice.geoIP%7Cext.centralauth.ForeignApi%7Cext.centralauth.centralautologin.clearcookie%7Cext.cx.contributions%7Cext.cx.entrypoints.contributionsmenu%7Cext.cx.eventlogging.campaigns%7Cext.cx.widgets.callout%7Cext.dismissableSiteNotice%2CeventLogging%2CnavigationTiming%2Cpopups%2CwikimediaEvents%7Cext.echo.api%2Cinit%7Cext.ores.highlighter%7Cext.uls.common%2Ccompactlinks%2Cinterface%2Cpreferences%2Cwebfonts%7Cjquery%2Cmoment%2Coojs%2Coojs-ui%2Coojs-ui-core%2Coojs-ui-toolbars%2Coojs-ui-widgets%2Coojs-ui-windows%2Csite%7Cjquery.client%2Ccookie%2ClengthLimit%2CmakeCollapsible%2CtextSelection%7Cjquery.uls.data%7Cmediawiki.ForeignApi%2CString%2CTitle%2CUri%2Capi%2Cbase%2Ccldr%2Ccookie%2Cexperiments%2Chtmlform%2CjqueryMsg%2Clanguage%2Cstorage%2Cuser%2Cutil%2Cwidgets%7Cmediawiki.ForeignApi.core%7Cmediawiki.editfont.styles%7Cmediawiki.htmlform.ooui%7Cmediawiki.libs.pluralruleparser%7Cmediawiki.page.ready%7Cmediawiki.special.contributions%2Crecentchanges%7Cmediawiki.ui.button%7Cmediawiki.widgets.DateInputWidget%2CUserInputWidget%7Coojs-ui-toolbars.icons%7Coojs-ui-widgets.icons%7Coojs-ui-windows.icons%7Coojs-ui.styles.icons-content%2Cicons-editing-advanced%2Cicons-movement%7Cskins.vector.legacy.js%7Cuser.defaults&skin=vector&version=wjef9
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Are you sure this is related to CX? At Logstash I see this on various pages and the error is happening inside the callstack of mw.user.sessionId().
Looking at the stack trace it occurs inside CXContributions.prototype.render
Given it occurs inside fire this tells me that somethig is subscribing to mw.hook( 'mw.cx.cta.shown' ) with a handler that throws the error. I don't know enough about what uses that code to say if this is CX specific.
Note URIError: malformed URI sequence is a very generic error message. It's caused when you try to decode badly encoded URIs. Either CX is passing a bad value or like you say there is an issue in mw.user.sessionId or somewhere deeper in the stack.
Likely similar to T324976: VE crashes on page with partially percent encoded URL in anchor of internal link and T286009: Unable to show preview on https://www.mediawiki.org/w/index.php?title=Help:Extension:Translate/Group_configuration&veaction=editsource. Anchors from wikitext can be invalid, so the uri handling should take that into account or it might throw a URIError.