I have the exact same issue:
Warning: Class ‘Tightenco\Collect\Support\Arr’ not found in ~/public_html/mysite/wp-content/plugins/wpdm-dropbox/libs/vendor/tightenco/collect/src/Collect/Support/alias.php on line 16
Parse error: syntax error, unexpected ‘=’ in ~/public_html/mysite/wp-content/plugins/wpdm-dropbox/libs/vendor/tightenco/collect/src/Collect/Support/Arr.php on line 388
