New error in v16 for uddtio

I have just installed v16 and code which compiled fine with all the previous v16 betas, v15 and v14 now no longer compiles. I get the following error message on lines of code that use UDDTIO, e.g.

    subroutine dump_strsum(f)
        character(len=*), intent(in) :: f
        integer :: u
        open(newunit=u,file=f,access='sequential',status='replace')write(u,'(dt)') strsum_data
        close(u)
    end subroutine dump_strsum

error #5521: A derived-type object in an input/output list cannot have inaccessible components unless a suitable user-defined input/output procedure is available.

As far as I can see all the components are public so I don't understand why this message has started appearing. Is there a new, more stringent, check? Is there anyway of working out what the actual error is? I have tried disabling error 5521 but that isn't allowed.

Thanks.