control: tags -1 confirmed I haven't explicitly reproduced this, but based on the description I am confident this is an issue.
What happened is that upstream dropped an internal symbol krb5int_c_combine_keys that is used by the old library. So once libk5crypto3 is upgraded, libkrb5-3 breaks. Interestingly it looks like libkrb5-3 from version 1.18 will work fine with the old libk5crypto3. A breaks relationship would certainly make things better. I'm not 100% sure it's good enough if krb5 is effectively in the pseudo-essential set. I asked on IRC and was confused by the answers I got. So I'm investigating.