Repro steps:

1. Open Jenkins master site
2. Sign in as admin
3. Open "Manage Jenkins" -> "Manage Plugins"
4. Click "Available" tabs
5. Type some text (example "docker") in the filter box in the upper right and press Enter

Expected result:
Plugins with "docker" in name shown

Actual result:
HTTP ERROR 403
Problem accessing /pluginManager/install. Reason:
No valid crumb was included in the request

Notes:
The filter box will eventually return plugins with searched for filter if you do not press enter, but the error should not occur if you do press enter.