Reasons for the error:
1. Incorrect content of the index.php or index.html file, its absence.
Solution: Open the file, carefully check if there are any errors in the syntax, if all the configuration parameters are set correctly.
ISP Manger allows you to edit files online, it is equipped with a language interpreter, so it will easily show you syntax errors.
If the file is not in the root directory of your web project, then create it. You can download it or create it using ISP Manager.
2. Inaccessibility of index.php or index.html due to incorrectly configured access rights.
Solution: You need to set 755 rights to the folder with your web project. This can be done in the IPS Manager through the file manager, by selecting the desired file and using the "Attributes" tab to set access rights, that is, the number 755.
3. You recently changed your DNS server
You will not be able to fix the error on your own. After the domain transfer, the information on the servers is updated. Updating DNS servers takes no more than one day, but more often the procedure takes from three to five hours.
4. You follow the link that is attached to a file that is deleted or inaccessible.
Check the existence of the file, its access rights.