Summary

New Spaces cannot be created on Confluence, using either the Blank space option or any of the default blueprints. 

 If this error is preventing new pages from being created as well, there is another similar problem that can cause this - please see:

• Error when starting or creating new content - ERROR: duplicate key value violates unique constraint "<tablename>_pkey"

Diagnosis

Checking the atlassian-confluence.log file, the following message (or similar) can be seen:

2020-05-20 16:06:32,009 ERROR [http-nio-9090-exec-12] [engine.jdbc.spi.SqlExceptionHelper] logExceptions ERROR: duplicate key value violates unique constraint "os_propertyentry_pkey"
Detail: Key (entity_name, entity_id, entity_key)=(confluence_ContentEntityObject, 0, confluence.inline.tasks.sequence.last) already exists.
– referer: https://confluence.url | url: /confluence/rest/create-dialog/1.0/space-blueprint/create-space | traceId: 06628717af98cdcb | userName: username

That error indicates there is a duplicate record on a particular table - this can vary depending on the error message, but it will generally indicate what is the affected table ("os_propertyentry") and the duplicate record ((entity_name, entity_id, entity_key)=(confluence_ContentEntityObject, 0, confluence.inline.tasks.sequence.last)).

Cause

There is a duplicate record (or multiple duplicates) on the table mentioned by the error message, which is preventing the new Space from being created, due to this problem on the database side.

Solution

Based on this information from the error message on the logs, run a query on the affected database table, to check for any duplicated records.

Following the example from the above log snippet:

select entity_name, entity_id, entity_key FROM OS_PROPERTYENTRY group by entity_name, entity_id, entity_key having count(*) > 1;

This will return all records on this table that have those duplicated values. For each of the duplicated records from the above query, run another select query, to find all records of that particular kind that are duplicates:

select * FROM OS_PROPERTYENTRY where
(entity_name = 'entity_name_from_above_query' and entity_id = 'entity_id_from_above_query' and entity_key = 'entity_key_from_above_query')

This will return all individual duplicate records. Please compare the duplicate records' values for all columns, as there can be two possible cases here:

1. There's a different value (or more) on some particular column
2. All values on all columns are identical

For the first case, where there is some differing value on a particular column, remove all of the duplicates except for one. As a general rule, keep the one record that has the highest value, on the column where the different values appear.

For the second case, it's not possible to remove just one of the duplicates, as we can only remove all of the records at once since they're identical. Remove all of the identical records, and then add one of them back manually to the table, afterwards.

Once the duplicated records are dealt with, start Confluence, and creating Spaces should be possible once more.