This issue occurs when the device has reached the limit to handle multiple processes on a given device.
For example, with approximately 400 users signed in and where each user is accessing web resources there can only be a maximum of 2 Web URL's or processes created by each user for the total processes to reach 800. Beyond that, only 200 additional Web processes would be allowed before the maximum is reached.
Therefore, if more than 2 Web requests are created by users at any given time, then the maximum number of rewrite-server processes allowed can easily be exceeded with 400+ Web users signed into the device.
To confirm the number of Web processes that are being consumed for any one user, check the user access log for multiple Web resources being accessed by the same user within the same time frame. Each unique Web URL accessed by any one user is an additional Web process.