Somewhat related to https://code.onedev.io/onedev/server/~issues/1066 I guess:

java.lang.NullPointerException
    	at io.onedev.server.entityreference.ReferenceMigrator.migratePrefixed(ReferenceMigrator.java:22)
    	at io.onedev.server.plugin.imports.youtrack.ImportServer.importIssues(ImportServer.java:1016)
    	at io.onedev.server.plugin.imports.youtrack.ImportServer.importProjects(ImportServer.java:1101)
    	at io.onedev.server.plugin.imports.youtrack.YouTrackProjectImporter.doImport(YouTrackProjectImporter.java:78)
    	at io.onedev.server.web.page.project.imports.ProjectImportPage$1$1$1.call(ProjectImportPage.java:79)
    	at io.onedev.server.web.page.project.imports.ProjectImportPage$1$1$1.call(ProjectImportPage.java:75)
    	at io.onedev.server.persistence.DefaultTransactionManager$1.call(DefaultTransactionManager.java:72)
    	at io.onedev.server.persistence.DefaultSessionManager.call(DefaultSessionManager.java:90)
    	at io.onedev.server.persistence.DefaultTransactionManager.call(DefaultTransactionManager.java:60)
    	at io.onedev.server.web.page.project.imports.ProjectImportPage$1$1.runTask(ProjectImportPage.java:75)
    	at io.onedev.server.web.component.taskbutton.TaskButton$2.call(TaskButton.java:173)
    	at io.onedev.server.web.component.taskbutton.TaskButton$2.call(TaskButton.java:144)
    	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
    	at io.onedev.server.security.SecurityUtils$1.run(SecurityUtils.java:367)
    	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
    	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
    	at java.base/java.lang.Thread.run(Thread.java:829)

Looking at the code (https://code.onedev.io/onedev/server/blob/58e48ce4930de11c99aedc79963ced58c1f08aee/server-plugin/server-plugin-import-youtrack/src/main/java/io/onedev/server/plugin/imports/youtrack/ImportServer.java#L1016-L1016)

					for (IssueComment comment: issue.getComments()) {
						comment.setContent(migrator.migratePrefixed(comment.getContent(), youTrackProjectShortName + "-"));
						dao.persist(comment);
					}

It seems that one of the comments of the issue is missing?

Again - it would be nice to log which issue was being imported and cause the issue