When Slack opens in a White Screen the problem is either a Cache issue or a Hardware Acceleration issue.
Mouse over the upper right hand corner and you will see the menu > Help > Select Troubleshooting > Clear Cache and Restart. If the White Screen appears again got Menu > Help > Troubleshooting > Toggle Hardware Acceleration > and Clear Cache and Restart again.
Hover over the top left
This is the menu you can access with the white screen