@cdorin, this is still an issue with the latest Version 3.17.17+190918. Can you reproduce?

@cdorin, I just faced the same issue with a new system when trying to update from LS4 RC 14 to LS 4.01, see screen shot. The error shows up after having entered a ComfortUpdate key for validation.
This is the full error message:
"you have an error, or a notice, inside your local installation of limesurvey. See :
{"success":false,"message":"The CSRF token could not be verified.","error":{"code":400,"type":"CHttpException","errorCode":0,"message":"The CSRF token could not be verified.","file":"\/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/framework\/web\/CHttpRequest.php","line":1375,"trace":"#0 \/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/framework\/base\/CComponent.php(561): CHttpRequest->validateCsrfToken(Object(CEvent))\n#1 \/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/framework\/base\/CApplication.php(212): CComponent->raiseEvent('onbeginrequest', Object(CEvent))\n#2 \/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/framework\/base\/CApplication.php(183): CApplication->onBeginRequest(Object(CEvent))\n#3 \/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/index.php(195): CApplication->run()\n#4 {main}","traces":[{"file":"\/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/framework\/base\/CComponent.php","line":561,"function":"validateCsrfToken","class":"CHttpRequest","type":"->","args":[{"sender":{"defaultController":"surveys","layout":"main","controllerMap":[],"catchAllRequest":null,"controllerNamespace":null,"name":"LimeSurvey","charset":"UTF-8","sourceLanguage":"en_us","localeClass":"LSYii_Locale","preload":["log"],"behaviors":[]},"handled":false,"params":null}]},{"file":"\/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/framework\/base\/CApplication.php","line":212,"function":"raiseEvent","class":"CComponent","type":"->","args":["onbeginrequest",{"sender":{"defaultController":"surveys","layout":"main","controllerMap":[],"catchAllRequest":null,"controllerNamespace":null,"name":"LimeSurvey","charset":"UTF-8","sourceLanguage":"en_us","localeClass":"LSYii_Locale","preload":["log"],"behaviors":[]},"handled":false,"params":null}]},{"file":"\/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/framework\/base\/CApplication.php","line":183,"function":"onBeginRequest","class":"CApplication","type":"->","args":[{"sender":{"defaultController":"surveys","layout":"main","controllerMap":[],"catchAllRequest":null,"controllerNamespace":null,"name":"LimeSurvey","charset":"UTF-8","sourceLanguage":"en_us","localeClass":"LSYii_Locale","preload":["log"],"behaviors":[]},"handled":false,"params":null}]},{"file":"\/var\/www\/vhosts\/my-system.host\/dsc.my-system.host\/ls4download\/index.php","line":195,"function":"run","class":"CApplication","type":"->","args":[]}]}}"

Please also assign this ticket to someone else since Louis has left.

This is still an issue at latest 3.22.12 version.

Fix committed to master branch: http://bugs.limesurvey.org/plugin.php?page=Source/view&id=29821

Fixed in Release 4.1.17+200414

fixed in 4.4.0 rc1