1. Go to Control Panel > Web Content
2. Add > Basic Web Content
3. Leave the Title field empty and publish the web content
The solution is to "backport" the changes of commit https://github.com/liferay/liferay-portal/commit/3647577b2b58d30b9e0cc1d46e7a70282c4e0415 from trunk, which is part of
LPS-30963 but is a standalone fix for this issue (the entire LPS-30963 is a big new feature which should not be entirely backported for a specific bugfix).