Form lock addon not working

I already have WPDM installed and working in the Pro version. I purchased and installed de Form Lock addon, but after instalation and checking this option to a package, when I publish and preview the page, te page shows a PHP error:
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Warning: Illegal string offset ‘html’ in C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\wpdm-form-lock\wpdm-form-lock.php on line 67

Warning: Cannot assign an empty string to a string offset in C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\wpdm-form-lock\wpdm-form-lock.php on line 67

Warning: Illegal string offset ‘html’ in C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\wpdm-form-lock\wpdm-form-lock.php on line 68

Fatal error: Uncaught Error: Cannot use assign-op operators with string offsets in C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\wpdm-form-lock\wpdm-form-lock.php:68 Stack trace: #0 C:\xampp\htdocs\eletrobrasrondonia\wp-includes\class-wp-hook.php(286): WPDM_FormLock->download_lock(”, Array) #1 C:\xampp\htdocs\eletrobrasrondonia\wp-includes\plugin.php(203): WP_Hook->apply_filters(”, Array) #2 C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\download-manager\libs\class.Package.php(890): apply_filters(‘wpdm_download_l…’, ”, Array) #3 C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\download-manager\libs\class.Package.php(216): WPDM\Package::activeLocks(Array) #4 C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\download-manager\libs\class.Package.php(1135): WPDM\Package->Prepare(10868, ‘ <div class=\\”r…’) #5 C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\download-manager\wpdm-functions.php(1283): WPDM\Package::fetchTemplate(‘ <div class=\\”r…’, Array, ‘page’) #6 C:\xampp\htdocs\eletrobr in C:\xampp\htdocs\eletrobrasrondonia\wp-content\plugins\wpdm-form-lock\wpdm-form-lock.php on line 68
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If anyone can help, please make contact.

Thanks.

The page I need help with: [log in to see the link]