Hi
I would like to add this as a bug too with the same problems.
A few days ago I upgraded to the latest version of WHMCS via Softaculous and now in the admin area I have blank tabs, blank text on buttons and blank labels.
I can send a screen shot if this would help
Regards,
Mark
[Update] In case anyone else has this problem I have managed to fix this by the following steps.
Following the advice given above sadly didn't work for me, however I replaced the entire /whmcs/admin/ folder with the latest version and it works again, so I guess there were differences in the CSS files, which don't get updated just copying across the /whmcs/admin/templates folder.
As this is only admin-area presentation files, your database remains untouched.