The conversion of the homepage begins. I created a development environment using XAMPP.
My Nextcloud runs with PHP 8.0, but Dokuwiki does not yet support this PHP version. However, the Apache web server can only support one PHP version (without further intervention).
A bit of research on the internet, however, brought me to the conclusion that I can work around this relatively easily. The keyword is
This statement within a „Directory“ entry solves the problem. If you want to know more details, take a look at here or also here.
The many activities probably overwhelmed the Pi at times. Data migration from Owncloud to Nextcloud, backup and migration of the homepage as parallel activities led to overheating. I solved the problem by adding a small heat pipe and removing the top panel.
Further analysis showed that my idea of relocating as many I/O operations as possible to an SSD connected to the Pi has achieved the goal, but also has side effects. The USB port heats up the interior of the PI (which I have in a case) excessively.
I didn't really like the original design (Dokuwiki) or the alternative Greensteel. That's fine for a wiki, but a homepage should be „nicer“. So I adapted the template and published it on GitLab. After a short review of the adjustments, I came to the conclusion to create a completely own template based on the starter template. These activities are now complete.