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Your LimeSurvey version: 6.2.9  LimeSurvey Professional
Own server or LimeSurvey hosting: Own server
Survey theme/template: all
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I upgraded to 6.2.9 yesterday. I noticed that the update did not bring with it my own themes from previous surveys on earlier versions of Limesurvey (which I have been using since 2012). When I try to edit the Teme in the old way under Configuration__Advanced__Themes I get the following message, which I do not understand. I would be very grateful for help in this matter.:
"500: Internal Server Error Twig\Environment::createTemplate(): Argument #1 ($template) must be of type string, null given, called in /limeservice/instances/20/limesurvey/application/core/LSETwigViewRenderer.php on line 518"

There was a major change in the survey templates/themes in version 3.x - www.limesurvey.org/manual/New_Template_System_in_LS3.x/en

Pre 3.x themes are no longer compatible.

Thanks tpartner. That's a shame as we can't edit the Themes via Configuration- Advanced-Themes. I get the message " "500: Internal Server Error Twig\Environment::createTemplate(): Argument #1 ($template) must be of type string, null given, called in /limeservice/instances/20/limesurvey/application/core/LSETwigViewRenderer.php on line 518"
But we've submitted a ticket for this as we are in fact CLoud hosted not on our own server as I wrongly said when I first posted this problem. Cheers