I had the same problem. After hours of searching I found the cause which was especially for my windows setting. The 13 LTS TYPO3 core has problems with detecting correct paths on Windows engines (which is case for user500665, too). More in detail it is the resolvement of backslashes. The according core class is typo3/cms-core/Classes/TypoScript/IncludeTree/SysTemplateTreeBuilder.php:237, the Function is handleSetInclude().

The solution would be to add the handling of $path with GeneralUtility::fixWindowsFilePath($path) - which is not implemented by the core team yet. The bug is known - see https://forge.typo3.org/issues/105713.