The full message:

UA SDK error (Opc.UA.ServiceResult=0x80170000) in 'static Session.Create'. OPC UA service result - {BadCertificateUriInvalid}. The URI specified in the ApplicationDescription does not match the URI in the certificate. BadCertificateUriInvalid
+ Connection attempt #3; last connected at 01/01/0001 00:00:00 (local); unconnected for 00:01:07.0630000.
+ This error can be reported by the OPC-UA server if the client application URI has changed but an old application instance certificate was kept. Try to remove and recreate the application instance certificate.
+ Application URI from the effective application manifest: uri:Laptop-Stijn:Arkite%20Workstation:1
+ Own certificate subject URL name from certificate sub-id "": urn:Laptop-Stijn:Arkite Workstation
+ The client method called (or event/callback invoked) was 'BrowseMultiple[1]'.

What about my other question on disabling the checks all together?

Upgrading from QuickOPC 5.63 to 5.80, the certificate rules seem to be more strict, I get this error:
UA SDK error (Opc.UA.ServiceResult=0x80170000) in 'static Session.Create'. OPC UA service result - {BadCertificateUriInvalid}. The URI specified in the
ApplicationDescription does not match the URI in the certificate. BadCertificateUriInvalid [...]

The mismatch seems to be there indeed, connecting to "opc.tcp://localhost:49320" does not match the certificate (this worked in version 5.63):
Server certificate:
X509v3 Subject Alternative Name:
URI:urn:Laptop-Stijn:Kepware.KEPServerEX.V6:UA%20Server, DNS:Laptop-Stijn

But connecting to "opc.tcp://Laptop-Stijn:49320" produces this error:
UA SDK error (Opc.UA.ServiceResult=0x808A0000) in 'DiscoveryClient.GetEndpoints'. OPC UA service result - Error establishing a connection: BadNotConnected. [...]

In an attempt to temporarily get around this, I tried below code, but it still doesn't accept the server certificate. What else needs to be set for it to ignore certificates?

var parameters = EasyUAClientCore.SharedParameters.EngineParameters;
var policy = UACertificateAcceptancePolicy.TestingInsecure;
parameters.CertificateAcceptancePolicy = policy;
parameters.HttpsCertificateAcceptancePolicy = policy;

So my questions are:
- How can I get the certificates working again?
- Can I (temporarily) disable certificate checks all together?

You are right: Lamar 7.0 stopped support for .Net framework, it was added again in 8.0, and removed again in 9.0 (wtf?), I did not notice that. I will try with the latest QuickOPC version.

It is actually like this (the DI abstraction has many different implementations, Lamar is just one of them):

My software => QuickOPC => OPCFoundation => Micrsoft.AspNetCore.Server.Kestrel => Microsoft.Extensions.DependencyInjection.Abstractions
My software => Lamar => Microsoft.Extensions.DependencyInjection.Abstractions

QuickOPC demands (via-via) a recent version of Microsoft.Extensions.DependencyInjection.Abstractions, which can only work with a recent version of Lamar (>=7.0). That recent version of Lamar no longer supports .Net framework.

So possible solutions are:
- QuickOPC to have lower (via-via) version restrictions on Microsoft.Extensions.DependencyInjection.Abstractions, so that I can keep on using an old version
- for me to ditch Lamar in favor of another DI implementation
- for me to upgrade to .Net 6-9

Neither is going to happen short-term I believe. I'm now testing with QuickOPC 2022.1, which is the latest one to work with old version of Microsoft.Extensions.DependencyInjection.Abstractions (5.0).

Microsoft.Extensions.DependencyInjection.Abstractions is the full name, sorry about the confusion.