Internal Server Error when logging into WP-Admin

Internal server errors are usually described in more detail in the server error log. If you have access to your server error log, generate the error again, note the date and time, then immediately check your server error log for any errors that occurred during that time period. If you don’t have access to your server error log, ask your hosting provider to look for you.

Have you tried:
– deactivating ALL (yes all) plugins temporarily to see if this resolves the problem (plugin functions can interfere). If this works, re-activate them individually (one-by-one) to find the problematic plugin(s).
– If you can’t get into your admin dashboard, try resetting the plugins folder by FTP. Sometimes, an apparently inactive plugin can still cause problems. Also remember to deactivate any plugins in the mu-plugins folder (if you have created such folder). The easiest way is to rename that folder to mu-plugins-old.
– switching to the unedited default Theme (Twenty Sixteen, etc.) for a moment using the WP dashboard to rule out any theme-specific issue (theme functions can interfere like plugins). If you don’t have access to your admin area, use FTP , or your web-host’s cPanel or whatever file management application your host provides (no Dashboard access required). Navigate to /wp-content/themes/ and switch to the default theme by renaming your current theme’s folder by adding “-old” to the end of the folder name. Alternately, you can remove other themes except the default theme. That will force your site to use it.