Today my Iffice 365 account is behaving strangely. Yesterday I was able to activate my wsp solution using activate button from the ribbon in solutions gallery.
Today I can not activate my wsp solution. It gives blank in popup dialog box. (As shown in below image)
When I click activate button, nothing happens.
Note: My user has full rights and it is site collection administrator.
There has been an outage listed in the admin portal for most of the week regarding sandboxed solutions:
SP73009 - Custom Solutions and Workflows - Service degradation Service degradation - Jul 27, 2016 3:50 PM
Current Status: We're continuing to perform a detailed analysis of the issue to determine the next steps.
User Impact: Users may be unable to activate or use SharePoint Online Sandbox Solutions. This may include event receivers, workflows, web parts, feature receivers, and InfoPath forms.
Scope of Impact: A few customers have reported this issue, and our monitoring indicates that any user attempting to activate or use this feature will be affected.
Start Time: Friday, July 22, 2016, at 11:30 PM UTC
Next Update by: Friday, July 29, 2016, at 8:00 PM UTC
Code in sandboxed solution has been depricated by Microsoft in SharePoint Online for some time now, see Deprication of custom code in sandboxed solutions.
So that might be a possible reason for what you are experiencing. That they are now enforcing the no-code-policy.
(Or it might be an ongoing temporary issue, see Eric Alexander's answer)
What you should do anyway is to convert your sandboxed solution to an add-in, most likely a provider-hosted add-in.