I believe I have the same problem.
For me, I'm using Multibranch configuration and the problematic branch name is "release/v3.21.192.0".
If I visit my branch page like below, Blue Ocean page shows successfully.
1. Load http://my-jenkins/jenkins/job/my-build/
2. Hit "release/v3.21.192.0"
2. Hit "Open Blue Ocean"
3. Hit "release/v3.21.192.0"
4. BO page for release/v3.21.192.0 branch is loaded successfully.
Current URL is https://my-jenkins/jenkins/blue/organizations/jenkins/my-build/detail/release%2Fv3.21.192.0/36/pipeline
But when I refresh the page. 400 bad request response is received.
If I replace %2F with / on my web browser, then the page is redirected to "https://my-jenkins/jenkins/blue/organizations/jenkins/my-build/detail/release%2Fv3.21.192.0/36/pipeline/".
(Please note that a trailing forward slash is added)
But at this time, the response is 403 Forbidden.
It seems this is because JSESSIONID cookie is missing in the redirection request.
I've been going through this problem for a long time.
It would be great if this problem can be solved soon or anyone can tell me any workaround.
I believe this has been fixed and released as part of
JENKINS-47073. Please reopen if that is not the case.