Upgrading 2.0.20 to 2.2.24

[felix at crowfix.com]

I have a seldom-used need to encrypt a few files, and the last time I did was on a gentoo system running 2.0.20.

    gpg -e <source >dest -r felix at crowfix.com

I have migrated the .gnupg dir to an Ubuntu 18.04 system running 2.2.24, and the gpg command seems to have mutated.  The gentoo 2.0.20 command can decrypt what the Ubuntu 2.2.24 command encrypts.  But the Ubuntu 2.2.24 command will not decrypt either what it just encrypted or what the gentoo 2.0.20 command encrypted:

    gpg: encrypted with 2048-bit ELG key, ID 18DCDD20A3362105, created yyyy-mm-dd
          "Felix Finch (Scarecrow Repairman) <felix at crowfix.com>"
    gpg: decryption failed: No secret key

The enceyption command also seems pickier:

    gpg: 18DCDD20A3362105: There is no assurance this key belongs to the named user
    sub  elg2048/18DCDD20A3362105 1999-12-06 Felix Finch (Scarecrow Repairman) <felix at crowfix.com>
     Primary key fingerprint: E987 4493 C860 246C 3B1E  6477 7838 76E9 182E 8151
           Subkey fingerprint: 1A59 C8A1 81FB 6780 641C  D17E 18DC DD20 A336 2105

    It is NOT certain that the key belongs to the person named
    in the user ID.  If you *really* know what you are doing,
    you may answer the next question with yes.

    Use this key anyway? (y/N)

Can someone offer an explanation so I don't have to dredge through a zillion changelogs to see why 2.2.24 is pickier?  What does it mean to say there is no secret key?

-- 
            ... _._. ._ ._. . _._. ._. ___ .__ ._. . .__. ._ .. ._.
     Felix Finch: scarecrow repairman & wood chipper / felix at crowfix.com
  GPG = E987 4493 C860 246C 3B1E  6477 7838 76E9 182E 8151 ITAR license #4933
I've found a solution to Fermat's Last Theorem but I see I've run out of room o