Same thing here on same level 
I also had it happen during the last Mongol mission and I used the same method of pausing/unpausing. After destroying the Barbican the errors stopped popping up but the bridge became unrepairable and I couldn’t finish the mission at all. Happens every single time without fail.
Also of note is that while the game tells me to repair the bridge there is never an objective icon on the map nor a bridge icon and in one attempt the bridge disappeared from the game entirely.
Update:
I managed to bypass whatever is causing the errors by only sending my Hui Hui Pao into the city and destroying the Barbican and Imperial Palace before completing the objective for sending my army in, only this caused another more helpful issue in which the bridge was never broken and I could just go straight to Xiangyang without any repairing.
Sorry for necroposting, but I’ve had this happen twice in a row, in the Bremule scenario. Just letting the devs know.
Thank you for letting us know! This is still proper advice that will help our team:
Happens every time I try to play a skirmish, all other game modes work fine and I have all mods disabled except advanced game settings.
Advanced Game Settings isn’t fully-compatible with the expansion. Try setting starting villagers to 5 and trying again (credit goes to the AGS mod creator, who suggested this already) - or disabling AGS.