Description of problem:

I ran dnf system-upgrade from F29 to F30 on a computer with a BIOS using the i686 rpms starting April 6. After the system upgrade completed and the system was restarted, I briefly saw an error ending increment.mod not found before grub started. grub had no Fedora menu entries. I ran most of the commands from 5.0.6 kernel entry for F29 in grub2/grub.cfg.rpmsave with the version switched to F30, and the kernel and system booted. When I first ran sudo grub2-mkconfig -o /boot/grub2/grub.cfg in konsole in Plasma, no Fedora kernel entries were added. /grub2/i386-pc/increment.mod didn't exist. /boot/grub2/i386-pc/blscfg.mod and other files in that directory had last modified dates of Sept 2016 which was around when I first installed F24 on the drive. grub2-common-1:2.02-75.fc30.noarch and grubby-8.40-30.fc30.i686 were the versions during the system upgrade.

I changed GRUB_ENABLE_BLSCFG to false in /etc/default/grub and I installed the grubby-deprecated as suggested in https://bugzilla.redhat.com/show_bug.cgi?id=1652806#c36. sudo grub2-mkconfig -o /boot/grub2/grub.cfg then added the Fedora kernel entries correctly. Thanks for the workaround.

After the F30 kernel was selected in grub by pressing enter or automatically after 5 seconds, my system has rebooted after 1-2 seconds at least 10 times out of more than 50 boots in the last week. No messages are usually shown even when I removed rhgb quiet from the kernel command line in grub2 before booting. 
The first and second times the reboots happened, messages appeared for less than a second and involved one referring to a magic number and possibly an error loading a file such as the initramfs or kernel. Today, I saw "fatal error: token too large, exceeds YYLMAX" 20+ times with More at the bottom of the screen. More of the same errors were shown when I scrolled down. I restarted with control-alt-del, and the system started normally the next boot. journalctl doesn't show anything on these boots that reboot. I'm unsure if these reboots are related to this grub issue in #1652806, but I hadn't seen them in F29 and earlier. I reported most of these issues at https://bugzilla.redhat.com/show_bug.cgi?id=1652806#c52 I filed this report since the reboots and errors I've had aren't mentioned in that report and might be a different problem.

Version-Release number of selected component (if applicable):
grub2-common-1:2.02-75.fc30.noarch
grubby-8.40-30.fc30.i686
kernel-0:5.0.6-300.fc30.i686
kernel-0:5.0.7-300.fc30.i686

How reproducible:
The reboots after the kernel is selected have occurred about 20% of boots in F30. The error messages were shown in 3 boots.

Steps to Reproduce:
1. On a system with a BIOS and i686 rpms installed, I ran 
sudo dnf system-upgrade download --refresh --releasever=30 --enablerepo=rpmfusion-free-rawhide,rpmfusion-nonfree-rawhide --setopt='module_platform_id=platform:f30' --setopt=install_weak_deps=False

sudo dnf system-upgrade download --refresh --releasever=30  --setopt='module_platform_id=platform:f30' should be enough
2. in the grub command prompt run most of the commands from latest kernel entry for F29 in grub2/grub.cfg.rpmsave with the F30 version instead.
set gfxpayload=text
insmod gzio
insmod part_msdos
insmod ext2
set root='hd0,msdos2'
linux   /vmlinuz-5.0.6-300.fc30.i686 root=/dev/mapper/fedora-root ro rd.lvm.lv=fedora/root rd.lvm.lv=fedora/swap rhgb quiet audit=1 audit_backlog_limit=8192 
initrd  /initramfs-5.0.6-300.fc30.i686.img

3. set GRUB_ENABLE_BLSCFG=false in /etc/default/grub
4. sudo grub2-mkconfig -o /boot/grub2/grub.cfg
5. sudo dnf install grubby-deprecated
6. sudo ls -l /usr/lib/grub/i386-pc/increment.mod
7. sudo ls -l /usr/lib/grub/i386-pc/blscfg.mod
8. sudo cp /usr/lib/grub/i386-pc/increment.mod /boot/grub2/i386-pc/
9. sudo cp /usr/lib/grub/i386-pc/blscfg.mod /boot/grub2/i386-pc/
10. reboot many times and select the latest kernel in grub
11. upgrade to the 5.0.7 kernel
12. reboot many times and select the latest kernel in grub

Actual results:
The system rebooted after the kernel was selected in grub in about 20% of cases in F30. Errors were shown in 3 of those boots.

Expected results:
No reboots or errors when loading the kernel from grub.

Additional info:

"fatal error: token too large, exceeds YYLMAX" was reported for RHEL at
https://bugzilla.redhat.com/show_bug.cgi?id=1239170

The boot entries in my /boot/grub2/grub.cfg look normal without excessively long strings as reported there.