seems like some users are getting hit by the issue coming up since last weekend
https://www.google.com/search?…st+external+ca+root+issue
german IT news outlet golem.de: https://www.golem.de/news/sect…r-aerger-2006-148840.html
a closer look on the first post plus the general IT news from all over the internet point to this exact issue.
It says "Addtrust" right there.

After 20 years(!), this root certifcate expired naturally and it is currently causing issues all over the internet because some clients rely on it.
This includes the Gameforge Client.
Colors added to the relevant parts of the error message.
Yellow = the name of the certificate in question, so you can see its the one the google link at the top of the post is talking about.
Red: The culprit for the error itself. May 30 2020 was ... last Saturday. 
As for a solution ... I'm not 100% sure yet.
Things worth trying:
a) deleting or deactivating the certificate in question on your machine. (It expired on May 30th anyway, so even if it does not fix the issue, it cant make it worse)
Run > certmgr.msc
see Trusted Root Certification Authorities > Certificates
find the "AddTrust External CA Root" and delete it. You can export it first via rightclick > all tasks > export , just in case.
This is probably not advisable it this section reads like Chinese to you 
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b) https://www.ssl.com/blogs/addt…root-expired-may-30-2020/ (See the part about replacement certificates)
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c) rather simple: pressing "Yes" when the question pops up again, because this is not an attack or anything but it's simply an expired root certificate - this is just a workaround.
As for a permanent solution that does not require manual fiddling by users:
I suspect the libeay32.dll from the Gameforge Client needs to be replaced with a newer version. We'd need a client version update.
I've reported the problem and its likely issue as I'm typing this so someone who works on the Client can look into it.
(OpenSSL 1.0.2.X is affected, and the file has either 1.0.2.15 or 1.0.2o )
d) (only semi-serious) .... switch back your system time to before May 30th? XD