This issue is caused by a bug in Sitefinity.

There is a duplicate entry for one of the dynamic content providers in sf_mb_dnc_cnt_prvdr_sf_prmssns and sf_mb_dnc_cnt_provider database tables.

This can be checked with the following query:

SELECT id, nme, * FROM sf_mb_dnc_cnt_provider
WHERE parent_secured_object_type = 'Telerik.Sitefinity.DynamicModules.Builder.Model.DynamicModuleType'
AND parent_secured_object_id IN (SELECT id FROM sf_meta_types WHERE name_space = 'Telerik.Sitefinity.DynamicTypes.Model.ModuleName') -- Replace with name of module

The query above should return at least 2 entries having the same name.
 

Fixed in version 15.2.8421.

Note: Before executing the below steps, create a backup of the database since the steps involve alteration of data.

Remove the duplicate entries and restart the application.

delete
from sf_mb_dnc_cnt_prvdr_sf_prmssns
where [id] in
(
    select id
    from sf_mb_dnc_cnt_provider
    inner join
        (
        select parent_secured_object_id, nme, count(*) as cnt, min(id) as minId
        from sf_mb_dnc_cnt_provider 
        group by parent_secured_object_id, nme
        having count(*) > 1
        ) as t1
    on
        sf_mb_dnc_cnt_provider.parent_secured_object_id = t1.parent_secured_object_id and 
        sf_mb_dnc_cnt_provider.nme = t1.nme
    where 
        sf_mb_dnc_cnt_provider.id <> t1.minId
)

delete sf_mb_dnc_cnt_provider 
from sf_mb_dnc_cnt_provider
inner join
    (
    select parent_secured_object_id, nme, count(*) as cnt, min(id) as minId
    from sf_mb_dnc_cnt_provider 
    group by parent_secured_object_id, nme
    having count(*) > 1
    ) as t1
on
    sf_mb_dnc_cnt_provider.parent_secured_object_id = t1.parent_secured_object_id and 
    sf_mb_dnc_cnt_provider.nme = t1.nme
where 
    sf_mb_dnc_cnt_provider.id <> t1.minId

sf_mb_dnc_cnt_prvdr_sf_prmssns is a relational database table that holds the connection between a provider and the permission sets associated with it. Those permission sets are kept in sf_permissions table and the details about the provider are kept in sf_mb_dnc_cnt_provider.

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