“PHP message:PHP Warning:Illegal string offset ‘form_id’

I get error 504 when go to User page.
I get below error logs.
I am a paid customer but I didn’t receive any reply from the ticket I created few days ago.
I have no choices, so I post my question here.
Please advise.

mywebsite@IeO-mywebsite:~/logs$ view error.log
2020/03/13 16:42:14 [error] 40245#40245: *450193 FastCGI sent in stderr: “PHP message: PHP Warning: Illegal string offset ‘form_id’ in /www/mywebsite/public/wp-content/plugins/ultimate-member/includes/um-short-functions.php on line 943PHP message: PHP Warning: Illegal string offset ‘form_id’ in /www/mywebsite/public/wp-content/plugins/ultimate-member/includes/um-short-functions.php on line 943PHP message:
PHP Warning: Illegal string offset ‘form_id’ in /www/mywebsite/public/wp-content/plugins/ultimate-member/includes/um-short-functions.php on line 943PHP message: PHP Warning: Illegal string offset ‘form_id’ in /www/mywebsite/public/wp-content/plugins/ultimate-member/includes/um-short-functions.php on line 943PHP message: PHP Warning: Illegal string offset ‘form_id’ in /www/mywebsite/public/wp-content/plugins/ultimate-member/includes/um-short-functions.php on line 943PHP message: PHP Warning: Illegal string offset ‘form_id’ in /www/mywebsite/public/wp-content/plugins/ultimate-member/includes/um-short-functions.php on line 943PHP message: PHP Warning: Illegal string offset ‘form_id’ in /www/mywebsite/public/wp-content/plugins/ultimate-member/includes/um-short-functions.php on line 943PHP message: PHP Warning: Illegal string offset ‘form_id’ in /www/mywebsite/public/wp-content/plugins/ultimate-member/includes/um-short-functions.php on line 943” while reading response header from upstream, client: 116.49.123.123, server: mywebsite.com, request: “POST /wp-admin/tools.php?page=acui HTTP/1.0”, upstream: “fastcgi://unix:/var/run/php7.3-fpm-mywebsite.sock:”, host: “mywebsite.com”, referrer: “https://mywebsite.com/wp-admin/tools.php?page=acui”