This happens because you have executed version 1.8.8 after running version 1.8.8-2. This problem should not happen under normal working conditions.
Do the following to solve this issue:
- From the main menu, select Resources > Updates > Manage Repositories.
- On the Manage Update Repositories dialog, click Default, then OK.
This will reset update repositories to the default ones required by the running PixInsight application.