Hello,
Please make sure that you are using the latest version of wpdm and all the addons of wpdm that you are using in your website.
Then if the issue persists, please check if php version of your website is minimum 7.4 . After that You may need to run a conflict test on your site to detect what actually causes the problem. if there are any conflicts among the installed plugins. To troubleshoot the issue, please deactivate other plugins except for WPDM with all addons of WPDM if you are using and check if the issue still persists. If that solves your problem, activate the plugins one by one, and continue checking the issue. In this way, you will be able to detect which one causes the problem.
Let us know for further assistance.
Regards
Tanvir