DNN 9.8 dnnModuleManager

You might want to take a look at the logs in portals/_default/logs.

For debugging, you might increate the logging level.

The error seems to appear once in a while but every time in context of bad html:
- https://www.dnnsoftware.com/forums/threadid/536066/scope/posts/threadpage/2 : old theme with tables.
- third party module not closing a div: https://github.com/dnnsoftware/Dnn.Platform/issues/3407

One more thing: As your error points to cookies and you mention 9.8....I remembered an issue with cookies that was solved for 9.8.1: https://github.com/dnnsoftware/Dnn.Platform/issues/4308

Thanks for your advice on the subject - I have been through the log files with a log4net setting of 'All' but apparently this type of Javascript error isn't registered at the backend level.
Since the pages are created using a template containing preembedded and empty Live Html modules it could be that some kind of 'bad html error' since the markup tends to somewhat complicated.
Another thing to mention regarding this issue is the fact that when adding a new module on a page deploying the errorprone template the module in question will not be visible before emptying the cache. So I guess updating dnn would be worthwhile

Some further investigation in this issue reveals that the problem arises, regardless of the chosen skin, but depends upon whether the page/tab in question has been added by means of apage template that was created with the option 'Include content' turned on.
That makes it three different factors having to be present in order to make the error occur:
Client Resource Management must be turned on
The page in question must visible in the menu
The page must be have been created from a dnn template utilizing the option 'Include content'

As mentioned earlier, this error appeared first time after upgrading to dnn 9.8 and I have seen in the changelog of the latter that a bug related to inserting af new module has been fixed as described in this url:
https://github.com/dnnsoftware/Dnn.Platform/issues/4112
It isn't exactly the same error, but both relate to the utilization of cookies in order to insert a new module. The inserted module in my case always ends up in the default contentPane and becomes visible once the dnn server cache has been emptied.
I pretend the problem could be related to the state of the recycle bin as this one is rarely if ever emptied and contains hundreds of soft deleted tabs and modules since it tends to occur on tabs having a specific parent page

I have had a similar issue and it was also related to HTML errors in the content of the placed module.
I would try uisng an HTML validator on the page after you cleared the Cache to see if that might be the case.