Odd to find this problem being so old, but I have some updates which might apply here . . .
I see this issue as described above in 1.458. However, I also have two systems one at 1.459 and one at 1.460 . . . in both these systems, when I mouse over a pending job that is waiting for an executor I see "Waiting for next available executor on <line noise>". (Line Noise == Huge long string of random characters that fills the screen to the right.)
Note that whatever this issue is, it is not isolated to the GUI. Both those systems, ONLY when there is a job waiting for a specific system, the JSON returned from a python/jenkins get_queue_info() returns as "malformed" error . . . I debugged into the JSON and see this exact same message (with line noise) shown in the malformed JSON packet . . . at exactly the place in the JSON that the error message said was malformed.
Like I say, for both the GUI and the JSON . . . this only happens when a job is waiting for a specific system . . . other types of jobs do no cause this problem. Unfortunately, we have a fair percentage of these on some systems.
I down graded the machine at 1.460 to 1.458 . . . now JSON works, but as noted above, the system name is now not shown . . . I'm guessing in one version you got lucky and are pointing at zero to terminate the string, but in the next two version you just happened to point at active data.
NOTE: I find this very easy to duplicate on my systems:
1. fill up all executors with any job (my favorite is sleep 1000000)
2. now queue any job that requires a specific system to run . . .
3. wait 15 seconds, then mouse over the waiting job.
P.S. This problem will duplicate as described even if ALL PLUGINS are disabled . . . it is not a plugin issue . . . at least not in 1.459 and/or 1.460 . . .