Hi,
The notice message with the TaxCloud plugin is not really an error. Just a check is missing to avoid the notice message but it doesn't change the behavior of the plugin and normally, the notices aren't supposed to be displayed on a live website for security reasons.
Still, I've added a patch to avoid that.
The fatal error comes from something trying to display an error message with a wrong formatting.
I've also added a patch on our end so that error messages with a wrong formatting are discarded to avoid similar fatal errors.
Donwload the install package on our website and install it on yours to get these patches.