If the problem occurred right after website builder installation or update, the reason may be in:

• Improper website builder version used during installation/update (there are two versions — for PHP 5.6 or lower and for PHP 7 or higher). To fix this problem make website builder manual update. Please find instruction on your On-Premises brand page ("Builder installation guide" -> "Update" section).
• Improper website builder archive extraction — possibly not all files were successfully extracted which lead to website builder crash.
  To fix this problem make website builder manual update.
• File ".htaccess" located in website builder web root directory. Possibly some directives (like "Options" or "DirectoryIndex") are not allowed by your hosting and it ends with error 500. Make sure that hosting settings let to use/overwrite these options.

Another reason can be logged in file "error_log" in web root folder of website builder. Please check if this file exists and that the new output is written once you get error 500. According to error, you can determine what causes this error to happen.