Problem solved.

I noticed the following at a customer:
- On the RDS server dbExplorer crashed
- On the Domain controller it ran well

But I also noticed that the domain controller showed this popup:

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This message appears when dbExplorer tries to show the Tip of the Day dialog
But I noticed that this message appeared right at the same moment where dbExplorer crashes on the other server.

So I turned off the tip of the day panel in the registry on the RDS server: problem gone.

For those of you with the same problem, change this in the registry:

Windows Registry Editor Version 5.00

[HKEY_CURRENT_USER\SOFTWARE\Data Access Worldwide\DataFlex Tools\19.1\DbExplorer\Preferences]
And for Vincent: please turn off this Tip of the day panel by default. I don't think anyone reads that. ;-)