If enabledevicedetection is ticked and devicedetectregex has any regular expression in it (and return value can be anything), then the Theme Selector page doesn't list any device or theme, and the following errors are shown:
Warning: array_keys() expects parameter 1 to be array, object given ___\moodle\lib\classes\useragent.php on line 216
Warning: array_merge(): Argument #2 is not an array in __\moodle\lib\classes\useragent.php on line 216
Warning: Invalid argument supplied for foreach() in __\moodle\theme\index.php on line 100
There are other errors reported on other pages too.
This seems to be the same as https://moodle.org/mod/forum/discuss.php?d=252721 (re: Moodle 2.6.1)