gitea: Error 500 creating a PR

• Gitea version (or commit ref): v1.15.1 (docker image from docker hub)
• Git version: N/A
• Operating system: debian 10
• Database (use [x]):
  • PostgreSQL
  • MySQL
  • MSSQL
  • SQLite
• Can you reproduce the bug at https://try.gitea.io:
  • Yes (provide example URL)
  • No
• Log :

2021/09/02 22:52:16 ...ervices/pull/pull.go:39:NewPullRequest() [I] [SQL] BEGIN TRANSACTION [] - 355.924µs
2021/09/02 22:52:16 models/index.go:92:getNextResourceIndex() [I] [SQL] SELECT max_index FROM issue_index WHERE
group_id = $1 [1] - 485.227µs
2021/09/02 22:52:16 models/index.go:66:GetNextResourceIndex() [I] [SQL] INSERT INTO issue_index (group_id, max_index) VALUES ($1,1) ON CONFLICT (group_id) DO UPDATE SET max_index = issue_index.max_index+1 [1] - 551.813µs
2021/09/02 22:52:16 models/pull.go:431:NewPullRequest() [I] [SQL] ROLLBACK [] - 107.085µs
2021/09/02 22:52:16 ...ers/web/repo/pull.go:1113:CompareAndPullRequestPost() [E] NewPullRequest: generate issue
index failed: pq: there is no unique or exclusion constraint matching the ON CONFLICT specification

Description

1. Created a branch locally

git checkout -b upgrade/mybranch

1. Pushed branch to gitea

git push origin upgrade/mybranch

1. On Gitea web app: create PR; error 500 occurs when committing the PR (clicking “Create Pull Request” button)

On a gitea running in docker; 1.15.0 used to work fine, but v1.15.1 does not.