It happens on both previous and current version (8.0.21.0_20180530).
The error alert is always the same: 
I'm on Win7x64, Office 365 Business, Excel Version 1805 (Build 9330.2087)
On trying to add connectors manually, putting i.e. GooglePlaces.xml into /connectors/ folder i fail too - added connector doesn't appear in any ribbon dropdown.
Hi,
Can you send me an email with a print screen of your "About" window in Seotools and a copy of the seotools.config.xml file? The path to the file should be available in the About window.
done - you've got email.
Thank you. I've replied to your email.
On beginning it seemed to work - on second try to run connectors manager - error. i mailed you.
After some tests i realized, that after start of Excel it is possible to run connector manager only once. If needing to run it twice, Excel should be restarted - than it is possible to run connector manager again, but even just only once. Also, once per Excel start.
you can update latest version and run on window and If any issue in your window then it is better to check https://babasupport.org/windows/windows-error-0x8024200b/ this for more information