Plesk shows the following error when attempting to unsuspend the customer:
Error: The account is still suspended due to the following reason: The account is suspended because its owner was suspended
"%plesk_dir%"\Mysql\bin\mysqldump.exe -uadmin -p<plesk_admin_password> -P8306 psa > backup_name.sql
mysql> update clients set status=0 where id in (select id from clients where login='problem_login');
After that it will be possible to suspend and unsuspend the customer without any issues from Plesk.
Note that you can suspend/activate the customer from command promt using Plesk command line utility
"%plesk_cli%\customer" --off <cutomer_name> "%plesk_cli%\customer" --on <cutomer_name>
More information about this utility you can find in Reference for Command-Line Utilities Guide.