Hi @mhhossein
You can try with two possible step.
First:
Go to your installation directory from cpanal (login cpanel–> file manager–> then you will find directory folder where wp installed)
Then go to wp-content/themes and plugins
Rename the plugin name and theme name
Now try again to login and if work then back to the plugin (actual plugin) one by one check which one causing problem.
Second:
If it deos not work update the wordpress core file.
Note: You should have backup to proceed WordPress core update.
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