Spam error

Hello,
I’ve got an error message in an orange box that says “There was an error trying to send your message. Please try again later.”.
I’ve already tried to set the theme to one of the original ones (like Twenty Seventeen) and deactivate all plugins except for Contact Form 7.
I also tried to copy it to another webserver, but there was no difference.
I looked up the developer tools and there I found out that the messages are marked as spam messages.

JSON:
into #wpcf7-f742-p30-o1
status spam
message There was an error trying to send your message. Please try again later.

Any suggestions?

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