This is a known issue in SharePoint. For that, a lot of people avoid custom lists in SharePoint (After using custom lists, some built-in functions may be restricted).
Please make sure you are not using special characters when working with custom lists.
You may run into this problem whether you are using a custom list or not. Avoid using special characters in the column names, such as #, @, %, etc. You may have to delete the column and recreate it without special characters before you can export the list to Excel.
In addition, a workaround is to:
Go to the Site Assets Library on the same site
Create an Excel file
Data > Get Data
Connect to SharePoint list
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