A change to how we parse in date range parameters (like "ThisWeek" or "NextMonth") has caused a problem.
If you have a Custom date range, that is stored in the system as a string like "2017/01/01-2017/01/31"
If you wanted it to be "all dates after 2017/01/01" then it would be stored as either "2017/01/01-" or "2017/01/01-null".
We added some extra functionality so that you could set up custom relative date ranges. Something like "6m-1w" would be interpreted as "from 6 months ago to 1 week's time".
Unfortunately, if the "null" value from "all dates after 2017/01/01" gets fed into the "parse this string as a custom duration" routine, it gets stuck and goes into an infinite loop.
We have a fix which we will be deploying later. The problem can be worked around by not running reports with a custom date parameter of "null", but because the system remembers the parameters you last used, if you ran a report with that parameter in the past, it will try to set it automatically on opening the report, and cause the lock up. In this case please contact us and we'll reset your stored default report parameters.