This bug is created on behalf of a Community post:
https://community.atlassian.com/t5/Confluence-questions/Unknown-Server-Error-in-Confluence-from-using-Atlassian-s-Space/qaq-p/1001852
Another community post having a similar issue:
https://community.atlassian.com/t5/Confluence-questions/Plugin-uninstall-destroys-Confluence/qaq-p/1019918#M135051

Issue Summary

When following the Confluence Space Blueprint tutorial, once you've created and applied your plugin. You're no longer able to create a Confluence Space.

Environment

Reported on Confluence 6.7.1, 6.12.2, 6.14.0 using Postgres 9.6.

(Optional - If Applicable)
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Steps to Reproduce

Steps to reproduce:

1. Install the latest Atlassian SDK as per https://developer.atlassian.com/server/framework/atlassian-sdk/install-the-atlassian-sdk-on-a-windows-system/
2. Navigate to the instruction set for creating a space blueprint: https://developer.atlassian.com/server/confluence/write-a-simple-confluence-space-blueprint/
3. Clone the repository located in those instructions that contains the completed space blueprint plugin to a folder on your local machine:

git clone git@bitbucket.org:atlassian_tutorial/confluence-space-blueprint.git

1. Open a command prompt (on windows) and navigate to the folder that you cloned the repository into.
2. Build and run the plugin by typing the following command into the command prompt:
   atlas-run
3. Open up a browser window and navigate to http://localhost:1990/confluence/ (or whichever link you use to access the local instance of confluence you just started).
4. Login using admin/admin as username/password.
5. Click on the Confluence Administration cog wheel in the upper right hand corner, and then click "Manage Apps."
6. Click on "Upload App."
7. Navigate to the directory you cloned the repository into, and click on the "target" folder (if this folder doesn't exist, you need to run atlas-run).
8. Select the space-blueprint-1.0-SNAPSHOT.jar file and click "Upload."
9. Refresh the page, and select "Create space" under the Spaces dropdown. Note that the Example Space Blueprint is listed as an option, and you can successfully create a space if you wish to test that.
10. Go back to Confluence Administration->Manage Apps, and uninstall the plugin you just installed.
11. Refresh the page, and select "Create space" under the Spaces dropdown. Note that no error occurs.
12. Select the command prompt, and hit Ctrl-C to kill the server instance currently running (you may have to navigate to Task Manager and kill a process named "Java SE Platform Binary" in order to completely kill the server).
13. After the server has stopped, type the following command into the command prompt:
    atlas-clean
14. Open pom.xml in your favorite text editor (or open as a project in IDEA or other IDE).
15. Change the confluence versions and data versions in the properties block from this (I also tried modifying my amps.version to 6.21, but it didn't fix anything):
    <properties>
    <confluence.version>6.7.1</confluence.version>
    <confluence.data.version>6.7.1</confluence.data.version>
    <amps.version>6.3.15</amps.version>
    <plugin.testrunner.version>1.2.3</plugin.testrunner.version>
    <atlassian.spring.scanner.version>1.2.13</atlassian.spring.scanner.version>
    <!-- This key is used to keep the consistency between the key in atlassian-plugin.xml and the key to generate bundle. -->
    <atlassian.plugin.key>${project.groupId}.${project.artifactId}</atlassian.plugin.key>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
    </properties>to this:
    <properties>
    <confluence.version>6.14.0</confluence.version>
    <confluence.data.version>6.14.0</confluence.data.version>
    <amps.version>6.3.15</amps.version>
    <plugin.testrunner.version>1.2.3</plugin.testrunner.version>
    <atlassian.spring.scanner.version>1.2.13</atlassian.spring.scanner.version>
    <!-- This key is used to keep the consistency between the key in atlassian-plugin.xml and the key to generate bundle. -->
    <atlassian.plugin.key>${project.groupId}.${project.artifactId}</atlassian.plugin.key>
    <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
    <maven.compiler.source>1.8</maven.compiler.source>
    <maven.compiler.target>1.8</maven.compiler.target>
    </properties>
16. Repeat steps 4 through 13.
17. Refresh the page.
18. Click on "Create Space" under the Spaces tab, and note the "Unknown Server Error" message you receive.
19. In the command prompt, look at the log output. You should see an error like the following:
    [INFO] [talledLocalContainer] 2019-02-07 15:35:17,015 WARN [http-nio-1990-exec-8] [confluence.impl.hibernate.ConfluenceHibernateTransactionManager] doRollback Performing rollback. Transactions:
    [INFO] [talledLocalContainer] ->[PluginReadWriteTx]: PROPAGATION_REQUIRED,ISOLATION_DEFAULT (Session #1567347778)
    [INFO] [talledLocalContainer] – referer: http://localhost:1990/confluence/plugins/servlet/upm | url: /confluence/rest/create-dialog/1.0/space-blueprint/dialog/web-items | traceId: 04182b73ba117a25 | userName: admin

Expected Results

You should be able to create a new space within Confluence.

Actual Results

When attempting to create a new space within Confluence, you get "Unknown Server Error"

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Notes

(Optional - If Necessary)

Workaround

No present workaround.