Connection timeout Nextcloud Client Linux

Hi there,

for some reason I had to manually renew my letsencrypt certificate.
Since then I always get an connection timeout error with my linux desktop client, the log gives me following message: OCC::WebFlowCredentials::stillValid Still valid?

but when I try to create a new account with the real ip Address of my server, the desktop client asks to trust the letsencrypt certificate, so it seems the server is reachable, there is only a problem with the ssl certificate.
Connecting with the browser is working fine.

Is there a way to reset or delete the old certificate from the client? Thanks
Iā€™m on manjaro linux 18.03 KDE
Nextcloud 15.04
Nextcloud client 2.5.1