Use the following sql for the Tabs table:

UPDATE Tabs SET StartDate=NULL WHERE StartDate LIKE '%9999%';
UPDATE Tabs SET EndDate=NULL WHERE EndDate LIKE '%9999%';

I am experiencing this in a DNN 6.1.1 instance, and have checked the DB and in the Modules table, all of the StartDate and EndDate values are already NULL. It happens that on a particular page, three different modules give this message when trying to load the settings for that module. other modules on the page work correctly.

In this case, the common element for the three modules with issues is that they are currently referenced by a zldnn module wrapper instance that is on a different page

i then made a test by taking two different modules that worked correctly allowing the settings to click/work, and on the zldnn module wrapper in the other location, had it wrap/include each of those working modules. With each test, as soon as the module was referenced by module wrapper, the settings started giving the 99999 error immediately. The extra fun was that when removing those modules from the module wrapper, they were still broken, giving the 99999 error.

I tried next deleting the instance of module wrapper, and can see its record gone from the zldnnmodulewrapper db table, however the original module still has the error after this step.

so not sure what the issue is here.