API error responses generated by AbuseFilter and SpamBlacklist have changed, as announced here: https://lists.wikimedia.org/pipermail/mediawiki-api-announce/2019-September/000148.html (relevant task: T229539).
It appears you have special handling for the old responses. It should be updated, otherwise they will probably show a generic error message instead of a more specific one.
extensions/MobileFrontend/src/mobile.editor.overlay/AbuseFilterOverlay.js: 9: * is related to the abusefilter extension. 17: className: 'overlay abusefilter-overlay' extensions/MobileFrontend/src/mobile.editor.overlay/AbuseFilterPanel.js: 7: * Panel that shows an error message related to the abusefilter extension. 36: readMoreMsg: mw.msg( 'mobile-frontend-editor-abusefilter-read-more' ) 44: <p></p><a href="#/abusefilter" class="readmore">{{readMoreMsg}}</a> 58: this.overlayManager.add( /^\/abusefilter$/, function () { 65: msg = mw.msg( 'mobile-frontend-editor-abusefilter-warning' ); 67: msg = mw.msg( 'mobile-frontend-editor-abusefilter-disallow' ); extensions/MobileFrontend/src/mobile.editor.overlay/EditorOverlayBase.js: 285: 'abusefilter-disallowed': 'extensionAbuseFilter', extensions/MobileFrontend/src/mobile.editor.overlay/parseSaveError.js: 48: if ( /^abusefilter-warning/.test( code ) ) { 51: type: 'abusefilter', 57: } else if ( /^abusefilter-disallow/.test( code ) ) { 60: type: 'abusefilter', 66: } else if ( /^abusefilter/.test( code ) ) { 69: type: 'abusefilter', extensions/MobileFrontend/src/mobile.editor.overlay/SourceEditorOverlay.js: 527: } else if ( data.type === 'abusefilter' ) { extensions/MobileFrontend/tests/node-qunit/mobile.editor.overlay/EditorGateway.test.js: 152: code: 'abusefilter-warning-usuwanie-tekstu', 162: code: 'abusefilter-disallow', 172: code: 'abusefilter-something', 465: type: 'abusefilter', 489: type: 'abusefilter', 513: type: 'abusefilter',