We have found an error specific to SSRS 2014 SP1 CU5 and higher. I am current at SSRS 2014 SP2 build 12.0.5511.0 (SP2CU1)
The issue does not happen in 2014 SP1 CU4 or lower, or in SSRS 2016.
The issue seems to only happen in Internet Explorer. I'm currently running 11.0.9600.18450. There is no problem when running SSRS from Chrome or Firefox.
This issue happens on all multi-valued parameters in all reports:
1. When you first open a report and drop down a multi valued parameter, the sizing handles are not on the bottom right of the drop down list, so you can not resize the list. You can, however, scroll and selected options.
2. After closing the drop down list and trying to open it a second time, the screen simply flashes (as if it's refreshing), but the drop down list will not drop down. The only way to get it to open a second time is to do one of the following:
a) re-open the report
b) refresh the page (which loses previously selected parameters)
c) select a different parameter that causes a screen refresh, which then allows the drop down list to open once, but again not a second time.
d) run the report (if the selected parameters are sufficient to allow the report to run). Again the drop down list will only open once.
We develop many reports for customers and can't control their version of SQL or which browser they use. Do recommending that they upgrade to SQL 2016 or that they user a specific browser is not an option.