Found the following (possibly) invalid URLs:
URL: https://mrgsolve.org/docs (moved to https://mrgsolve.org/docs/)
From: DESCRIPTION
man/mrgsolve_package.Rd
Status: 301
Message: Moved Permanently
For content that is 'Moved Permanently', please change http to https,
add trailing slashes, or replace the old by the new URL.
Found this with
R CMD check --as-cran