I have a new TYPO3 9.5.15 installation and added gridelements 9.5.0 and t3sbootstrap 4.4.5 to make content elements with 3 columns in a own 1 column backendlayout which is defined in PageTsConfig.
In older TYPO3 versions, this is working fine (i.e TYPO3 7) but in TYPO3 9 I get the following error in FE:
Tried resolving a template file for controller action "Standard->three_columns" in format ".html", but none of the paths contained the expected template file (Standard/Three_columns.html). The following paths were checked: xxx/typo3/sysext/fluid_styled_content/Resources/Private/Templates/, xxx/typo3conf/ext/gridelements/Resources/Private/Templates/, xxx/typo3conf/ext/t3sbootstrap/Resources/Private/Templates/Content/, xxx/typo3conf/ext/t3sbootstrap/Resources/Private/Templates/, ,
I've double checked and can confirm, that both, gridelements and t3sbootstrap are included in the roottemplate of the website.
As far as I found out, the templates are available in xxx/typo3conf/ext/t3sbootstrap/Resources/Private/Templates/Gridelements/ but are called ThreeColumns.html
Has anyone an idee what causes the error and how to resolve it?
Thanks in advance
At a first look it seems that there are 2 problems with your combination here:
The message "Standard->three_columns" points to a wrong static template, so I guess you included the new DataProcessing static of Gridelements, while t3sbootstrap depends on the old Plugin and USER based static.
The root path xxx/typo3conf/ext/t3sbootstrap/Resources/Private/Templates/Gridelements/ is missing in your setup.
At a second look it might even be enough to change the Gridelements static, since t3sbootstrap takes care of the root path based on that setup.