I just had a repeat of this issue, 
- identical box
- no 'OK' box, the window is just sitting there
- title isn't even visible in window & install questions haven't started...
- no crash files in /var/crash

I'm suspicious this issue isn't with ubiquity itself (assuming today is the same issue again), but the box itself.
In `dmesg` I see the following


[  484.289578] EIP: 0xb7f9fbb5
[  484.289581] Code: Bad RIP value.
[  484.289583] EAX: ffffffda EBX: ffffff9c ECX: 00610670 EDX: 00008000
[  484.289585] ESI: 00000000 EDI: b7a20000 EBP: 00000000 ESP: bff5efa0
[  484.289587] DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b EFLAGS: 00000293
[  484.289620] INFO: task blkid:2066 blocked for more than 362 seconds.
[  484.289623]       Not tainted 5.4.0-42-generic #46~18.04.1-Ubuntu
[  484.289626] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  484.289628] blkid           D    0  2066   2064 0x00000000
[  484.289631] Call Trace:
[  484.289636]  __schedule+0x292/0x800
[  484.289640]  schedule+0x2e/0xa0
[  484.289643]  schedule_preempt_disabled+0xd/0x10
[  484.289646]  __mutex_lock.isra.9+0x207/0x490
[  484.289650]  __mutex_lock_slowpath+0x12/0x20
[  484.289653]  mutex_lock+0x2f/0x40
[  484.289655]  __blkdev_get+0x70/0x4d0
[  484.289658]  blkdev_get+0xcd/0x110
[  484.289661]  ? bd_acquire+0x85/0xc0
[  484.289663]  blkdev_open+0x7d/0x90
[  484.289666]  do_dentry_open+0x1ca/0x3b0
[  484.289668]  ? blkdev_get_by_dev+0x40/0x40
[  484.289671]  vfs_open+0x25/0x30
[  484.289674]  path_openat+0x29c/0x1290
[  484.289679]  ? ring_buffer_lock_reserve+0x112/0x380
[  484.289682]  do_filp_open+0x6a/0xd0
[  484.289686]  ? __alloc_fd+0x36/0x170
[  484.289690]  do_sys_open+0x1ad/0x2c0
[  484.289693]  sys_openat+0x1b/0x20
[  484.289696]  do_fast_syscall_32+0x7f/0x240
[  484.289699]  entry_SYSENTER_32+0xaf/0x102
[  484.289701] EIP: 0xb7f99bb5
[  484.289704] Code: Bad RIP value.
[  484.289706] EAX: ffffffda EBX: ffffff9c ECX: 0109b490 EDX: 00088000
[  484.289708] ESI: 00000000 EDI: df30ac11 EBP: 0109ed50 ESP: bfb6db80
[  484.289710] DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b EFLAGS: 00000246
[  605.122403] INFO: task systemd-udevd:1069 blocked for more than 483 seconds.
[  605.122412]       Not tainted 5.4.0-42-generic #46~18.04.1-Ubuntu
[  605.122415] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
[  605.122418] systemd-udevd   D    0  1069   1057 0x00000104
[  605.122423] Call Trace:
[  605.122436]  __schedule+0x292/0x800
[  605.122443]  ? hrtimer_interrupt+0x12f/0x280
[  605.122448]  schedule+0x2e/0xa0
[  605.122451]  schedule_preempt_disabled+0xd/0x10
[  605.122455]  __mutex_lock.isra.9+0x207/0x490
[  605.122461]  __mutex_lock_slowpath+0x12/0x20
[  605.122463]  mutex_lock+0x2f/0x40
[  605.122467]  __blkdev_get+0x70/0x4d0
[  605.122470]  blkdev_get+0xcd/0x110
[  605.122473]  ? bd_acquire+0x85/0xc0
[  605.122475]  blkdev_open+0x7d/0x90
[  605.122478]  do_dentry_open+0x1ca/0x3b0
[  605.122481]  ? blkdev_get_by_dev+0x40/0x40
[  605.122483]  vfs_open+0x25/0x30
[  605.122487]  path_openat+0x29c/0x1290
[  605.122492]  do_filp_open+0x6a/0xd0
[  605.122496]  ? __alloc_fd+0x36/0x170
[  605.122500]  do_sys_open+0x1ad/0x2c0
[  605.122503]  sys_openat+0x1b/0x20
[  605.122507]  do_fast_syscall_32+0x7f/0x240
[  605.122510]  entry_SYSENTER_32+0xaf/0x102
[  605.122513] EIP: 0xb7f8bbb5
[  605.122520] Code: Bad RIP value.
[  605.122522] EAX: ffffffda EBX: ffffff9c ECX: 0073a470 EDX: 000a8800
[  605.122524] ESI: 00000000 EDI: 005509af EBP: 0000000e ESP: bff31e70
[  605.122526] DS: 007b ES: 007b FS: 0000 GS: 0033 SS: 007b EFLAGS: 00000246