I can’t really help when it comes to Nextcloud on web hosting, and I wouldn’t recommend it in general.
If you’re going to install Nextcloud on a web hosting platform, it’s usability and maintainability, depends almost entirely on what the hosting provider offers in terms of configuration options. Also, at the end of the day, it isn’t that much easier to manage a Nextcloud on a web hosting platform, than maintaining a manual installation on a VPS. Sure you have a web panel to manage everything, but these panels are often quite limited, and ultimately, you still need to know what all those option mean, and how to configure them for Nextcloud.
However, according to the website, they also seem to offer Nextcloud as a service. So why not just use that, instead of installing it yourself on their web hosting platform?
I don’t know anything about the specific situation in Japan, but I doubt that web hosting works fundamentally different in Japan than here in Europe. If you look through the forums, you’ll notice, that European users are facing similar issues on shared web hosting platforms. And of course, hosted Nextcloud instances, VPSs, or web hosting plans on dedicated servers, are also significantly more expensive in Europe, compared to shared hosting plans.
After all Netxloud has become a more and more complex piece of software over the years, and especially the requirements for additional features like Talk, Office etc. are simply too high, for many of the cheap shared hosting plans. That doesn’t necessarily mean it won’t work at all, but sometimes performance is bad or unpleasant workarounds are required in order to get things working, and some things might not work at all, depending on the specific hosting plan you’re using.
So, these issues are not likely to be typical for Japan, but they are typical for shared hosting platforms in general. And they are also kind of hard to support in a community forum, especially if the user that is trying to help, doesn’t use the same provider you are using… or like me, doesn’t use web hosting at all.
For me that means that you can change things below the line. There was just an issue with the .htaccess, if you have modifications, during an update, the file will be replaced and you have to do your modifications again. Not really sure if that was your issue.
For internal server errors, you should check the logfiles, if you have access, what might be wrong.
With webhosting, some seem to work and it is good. Others just have settings that are not really compatible with the use of Nextcloud and as a user you can’t change the configuration to make it work. Problem on top, often you don’t have access to logfiles and tools to analyze problems. Despite the task is much harder to analyze than on your own server, these users are generally less experienced.
In that case either ask them to help, or take a rental server instead. This will be easier and cheaper!