this is an odd one... I attack Umbar's last city. and then after tactical combat city and army are no longer selectable.. repeatable.. however after auto resolve city is mine and all is well.. also repeatable. This one i thought you really needed to see..
before tactical attack:

save file here
after tactical attack:

save file here
after auto resolve all is well (not doing image for this as its fine heh heh)
save file here