Hi Vitalii,
There is an automated process in Gluu server which automatically deletes the expired clients. As mentioned by Mohib, Dynamically registered clients will expire in 24 hours.So the clients you had registered are no more available in Gluu server.
To resolve this issue, you need to follow below steps
1- Login to your Wordpress as Admin ( Using Wordpress login not OpenId Connect)
2- Open OpenId Connect > Delete the existing Registration
3- DO a new Client registration like you had done before in WP
4- Go to your Gluu OP host as admin.
5- Navigate to OpenId Connect> Clients > Select your client
5- Scroll down to the end and change "Client Secret Expires" date value to a maximum value
Regarding "Is there some way to set expiration date to never?" you can not set expiration date to never, however you can set the value to maximum date you want.
Kind Regards,
Jajati