There are a number of things which are stopping this from working in qgis2web:
The layer group - these are not well supported in qgis2web, so I would remove all layer groups (the one in this project does not seem necessary)
Quick Map Services layers are not supported
Issues 1 and 2 can be solved by removing the layer group and QMS layers, and instead adding a Tile Server (XYZ) layer.
Inverted polygon renderers are not supported - there is no workaround for this at this time
The labelling expression contains functions which are not yet supported, though this is an area which is under active development, so progress might be made on this soon.
In summary, you have used various features of QGIS which qgis2web simply does not support. Sorry not to have better news.
ORIGINAL (SECONDARY) ISSUE:
This is a bug in the qgis2web code relating to error reporting. It is meant to print meaningful diagnostic information for the issue which has occurred. However, in some situations (specific operating systems, QGIS Python console not open) it instead results in the error reported above. This is completely unhelpful, since it replaces the meaningful error message with a message relating to the process of gathering the error message.
I've just committed a change to the code which will solve this problem. Error messages will now be logged to a dedicated qgis2web error log, rather than printed out to the QGIS Python console. Please download the master branch from Github (https://github.com/tomchadwin/qgis2web) and overwrite your qgis2web installation. A new version of qgis2web will be released soon which includes this fix.
When an error occurs, open the QGIS Log Messages Panel (in QGIS,
Log Messages Panel), and click on the
qgis2web tab. You can then report the error you find there, which should be specific to the issue you've encountered.