This is due to a component dll (AvalonEdit.dll) conflict with Revit Host, we did not found a reliable way of reproducing it untill recently. Fix has been applied to Dynamo4Revit 2016 and Dynamo4Revit 2017 today. Please try uninstall and reinstall the newer builds starting from tomorrow. Thanks for sharing your issue with us! Really appreciated it.
I am also having the same issue; I went through the process of reinstalling the program as well, nothing. I thought it was related to a package I may have downloaded, but there was no change. I have only been able to add numerical text, but no luck in adding any alphabetical text.
I got the issue on both my laptop and my workstation.
Once I got it corrected by complete uninstall of Dynamo and reinstall of latest daily build, but then I occurred again after a reboot.
The only viable solution I have found so far, was to completely uninstall Dynamo and then reinstall v.1.1, which seems to be working on both my workstation and my laptop.
I am having this issue too when upgrading to 1.2.1 stable build in both Revit 2016 and 2017. Disabling the ICSharpCode.AvalonEdit.dll seems to have fixed the issue.