I see my own reply up above here, but suddenly I have the issue again:
Server access Error: sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilderException: unable to find valid certification path to requested target url=https://repo.woodenstake.se/content/groups/All/se/hedefalk/lift-utils_2.5_2.11/0.1/lift-utils_2.5_2.11-0.1.pom
Even though I follow my own instructions above of importing isrgrootx1 and letsencryptauthorityx1, the problem persists. The only thing I can think of is that I have gone to using SAN extensions for all my subdomains in one cert. Or that the root cert has changed?