Issue Let's Encrypt: Internal error ;-P

[cat24max]

Hey guys,

since this morning I am unable to issue any new LE certificates. It always loads for about 30-40 seconds, then end with: Internal error ;-P

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<h1>Server Error</h1>
<div class="error-code">500</div>
<h2>Internal Server Error</h2>
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The server encountered an internal error or misconfiguration and was unable to complete your request.
Please contact the server administrator to inform of the time the error occurred and of anything you might have done that may have caused the error.
More information about this error may be available in the server error log.
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This happens to multiple subdomains for multiple domains. I have no idea what is happening. The web page opens up without any issue, even SSL (with certificate warning, though).
There is no PHP error in any of the log files...

How can I fix this?

(Using Win 2012, Plesk 17.5.3, IIS 8)