I think I have found the real culprit behind this problem. The undo-tree package
saves its history in a very direct form which looks like this:
"5c22c12309909a63b7377b79cf2526b92d756854"
[cl-struct-undo-tree [nil ([nil (#8=[nil nil ((19641 . 19642) (#(" " 0 1 (syntax-type string fontified nil)) . -19641) (undo-tree-id0 . -1) (19541 . 19547) (#(" " 0 1 (syntax-type string fontified nil)) . -19541) (undo-tree-id1 . -1) (19469 . 19493) (#(" " 0 3 (syntax-type string)) . -19469) (undo-tree-id2 . -3) (19330 . 19331) (#(" " 0 1 (syntax-type string)) . -19330) (undo-tree-id3 . -1) (18803 . 18804) (#(" " 0 1 (syntax-type string fontified nil)) . -18803) (undo-tree-id4 . -1) (18365 . 18366) (#(" " 0 1 (syntax-type string)) . -18365) (undo-tree-id5 . -1) (16435 . 16442) (#(" " 0 1 (syntax-type string)) . -16435) (undo-tree-id6 . -1) (16250 . 16256) (#(" " 0 1 (syntax-type string)) . -16250) (undo-tree-id7 . -1) (14242 . 14253) (#(" " 0 2 (syntax-type string)) . -14242) (undo-tree-id8 . -2) (30156 . 30172) (#(" " 0 2 (syntax-type string fontified nil)) . -30156) (undo-tree-id9 . -2) (26832 . 26864) (" " . -26832) (undo-tree-id10 . -4) (25460 . 25468) (" " . -25460) (undo-tree-id11 . -1) (25404 . 25412) (#(" " 0 1 (fontified nil)) . -25404) (undo-tree-id12 . -1) (25265 . 25305) (" " . -25265) (undo-tree-id13 . -5) (25195 . 25227) (" " . -25195) (undo-tree-id14 . -4) (25078 . 25118) (" " . -25078) (undo-tree-id15 . -5) (25009 . 25049) (" " . -25009) (undo-tree-id16 . -5) (24887 . 24895) (#(" " 0 1 (syntax-type string)) . -24887) (undo-tree-id17 . -1) (24102 . 24110) (#(" " 0 1 (syntax-type string)) . -24102) (undo-tree-id18 . -1) (24038 . 24046) (#(" " 0 1 (syntax-type string)) . -24038) (undo-tree-id19 . -1) (23986 . 23994) (#(" " 0 1 (syntax-type string)) . -23986) (undo-tree-id20 . -1) (23907 . 23914) (#(" " 0 1 (syntax-type string)) . -23907) (undo-tree-id21 . -1) (23845 . 23853) (#(" " 0 1 (syntax-type string)) . -23845) (undo-tree-id22 . -1) (23808 . 23815) (#(" " 0 1 (syntax-type string)) . -23808) (undo-tree-id23 . -1) (17359 . 17375) (#(" " 0 2 (syntax-type string)) . -17359) (undo-tree-id24 . -2) (16588 . 16596) (#(" " 0 1 (syntax-type string)) . -16588) (undo-tree-id25 . -1) (16532 . 16540) (#(" " 0 1 (syntax-type string)) . -16532) (undo-tree-id26 . -1) (15993 . 16001) (#(" " 0 1 (syntax-type string)) . -15993) (undo-tree-id27 . -1) (15944 . 15952) (#(" " 0 1 (syntax-type string)) . -15944) (undo-tree-id28 . -1) (15918 . 15926) (#(" " 0 1 (syntax-type string)) . -15918) (undo-tree-id29 . -1) (7553 . 7561) (#(" " 0 1 (syntax-type string)) . -7553) (undo-tree-id30 . -1) (7467 . 7475) (#(" " 0 1 (syntax-type string)) . -7467) (undo-tree-id31 . -1) (2516 . 2524) (#(" " 0 1 (fontified t syntax-type string face whitespace-space-after-tab)) . -2516) (undo-tree-id32 . -1) (2461 . 2469) (#(" " 0 1 (fontified t syntax-type string face whitespace-space-after-tab)) . -2461) (undo-tree-id33 . -1) (2317 . 2325) (#(" " 0 1 (fontified t syntax-type string face whitespace-space-after-tab)) . -2317) (undo-tree-id34 . 1) (undo-tree-id35 . 1) (undo-tree-id36 . -1) (1344 . 1352) (#(" " 0 1 (fontified t face whitespace-space-after-tab)) . -1344) (undo-tree-id37 . 1) (undo-tree-id38 . 1) (undo-tree-id39 . -1) (1009 . 1017) (#(" " 0 1 (fontified t face whitespace-space-after-tab)) . -1009) (undo-tree-id40 . 1) (undo-tree-id41 . 1) (undo-tree-id42 . -1) (#(" " 0 1 (syntax-table #7=#^[nil #^[#2=(0) nil syntax-table #5=
#^^[3 0 #1=(1) #1# #1# #1# #1# #1# #1# #1# #1# #2# #2# #1# #2# #2# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #1# #2# #1# (7) #1# #3=(2) #3# #4=(3) #1# (4 . 41) (5 . 40) #4# #4# #1# #4# #1# #4# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #1# #1# #4# #4# #4# #1# #1# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# (4 . 93) (9) (5 . 91) #1# #4# #1# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# #3# (4 . 125) #4# (5 . 123) #1# #1#] #^^[1 0 #^^[2 0 #5#
...
I spare the rest, because the crash is happening while reading the form starting at [cl-struct-undo-tree ...
Actually, the point is near the end of my lines on #^^[1
Emacs shouldn't crash while reading data it has been able to write (well, at least I would expect it).
Could it be that the syntax-table structure has been changed recently in a way that makes it impossible
to read one that has been printed with a previous version of emacs?