Is it possible that these users had images in their HTML signatures?

Try doing in mysql:

alter table sogo_user_profile MODIFY c_defaults LONGTEXT;
alter table sogo_user_profile MODIFY c_settings LONGTEXT;

Hi ludovic,
I see : yes, there is images. I understand that this can overfow some fields, but I would want to understand the relationship, if I can.
If the images were too big, the settings would not be recorded at all. Also if we modify the settings, say for example, add a sieve filter which make the field exceeding its size. But, what can cause this disappearance, randomly, without any modification ?

Data being truncates means the JSON payload isn't valid anymore, thus it is invalidated.

Hi Ludovic,
I made the change of the field types 2 weeks ago, unfortunatly one user tell us this morning that he have lost his signature and his settings.
I reopen the bug because I think I've found the source error, there may be a check or a workaroud to find for you, to avoid this case.

At 4:00AM, exactly when the phone of user concerned made an activesync connection, sogo overflows in memory. The problem is that there is absolutly no activity on the server at this time, just some terminals like this phone which make some ping requests one time a minute. So I don't understand why sogo has reached this limit, when it have no problem with it at work hours when about 40 terminals are connected and are working !!

sogo.log:

Sep 06 04:12:11 sogod [16246]: 1.2.3.4 "POST /SOGo/Microsoft-Server-ActiveSync?User=myuser&DeviceId=BC6A29MD9D3JHFP5KTA1HRLK0O&DeviceType=iPhone&Cmd=Ping HTTP/1.1" 200 13/0 10.722 - - 0
Sep 06 04:12:11 sogod [16246]: |SOGo| terminating app, vMem size limit (512 MB) has been reached (currently 513 MB)
<data>:117: element div: validity error : ID divtagdefaultwrapper already defined
<div id="divtagdefaultwrapper">
^
<data>:476: element div: validity error : ID editorUserSignature already defined
<div id="editorUserSignature">
^
<data>:1: element hr: validity error : ID zwchr already defined

</div></div></div></div></div></div></div></div><div> </div><hr id="zwchr"
^
<data>:1: element span: validity error : ID OBJ_PREFIX_DWT1198_com_zimbra_phone already defined
br><br><span class="Object" role="link" id="OBJ_PREFIX_DWT1198_com_zimbra_phone"
^
<data>:1: element div: validity error : ID yui_3_16_0_ym19_1_1499153001738_22463 already defined
i de juin.</span></div><div id="yui_3_16_0_ym19_1_1499153001738_22463" dir="ltr"
^
<data>:1: element div: validity error : ID yui_3_16_0_ym19_1_1499153001738_22463 already defined
dir="ltr"> </div><div id="yui_3_16_0_ym19_1_1499153001738_22463" dir="ltr"
^
<data>:1: element div: validity error : ID yui_3_16_0_ym19_1_1493810811789_2492 already defined
1493810811789_2492">Bonjour,</div><div id="yui_3_16_0_ym19_1_1493810811789_2492"
^
<data>:1: element div: validity error : ID yui_3_16_0_ym19_1_1493810811789_2492 already defined
10.04.17 par mail.</div><div id="yui_3_16_0_ym19_1_1493810811789_2492" dir="ltr"
^
<data>:1: element div: validity error : ID yui_3_16_0_ym19_1_1493810811789_2492 already defined
mois d'avril 2014?</div><div id="yui_3_16_0_ym19_1_1493810811789_2492" dir="ltr"
^
<data>:1: element div: validity error : ID yui_3_16_0_ym19_1_1493810811789_2492 already defined
ltr">Cordialement.</div><div id="yui_3_16_0_ym19_1_1493810811789_2492" dir="ltr"
^
<data>:1: element div: validity error : ID yui_3_16_0_1_1497521737805_2599 already defined
vril et mai 2017</span></div><div id="yui_3_16_0_1_1497521737805_2599" dir="ltr"
^
<data>:1: element div: validity error : ID yui_3_16_0_1_1497521737805_2599 already defined
_2599" dir="ltr"> </div><div id="yui_3_16_0_1_1497521737805_2599" dir="ltr"
^
<data>:433: element table: validity error : ID already defined
lpadding="0" border="0" bgcolor="#EEEEEE" style="background: #FFFFFF;" dir="rtl"
^
Sep 06 04:12:11 sogod [16571]: <0x0x7f9032ad50e0[WOWatchDogChild]> child 16246 exited
Sep 06 04:12:12 sogod [16571]: <0x0x7f9032858eb0[WOWatchDog]> child spawned with pid 19216
Sep 06 04:12:14 sogod [19216]: <0x0x7f9032bd4120[WOHttpAdaptor]> notified the watchdog that we are ready
Sep 06 04:12:14 sogod [19216]: <0x0x7f9032897830[SOGoCache]> Cache cleanup interval set every 300.000000 seconds
Sep 06 04:12:14 sogod [19216]: <0x0x7f9032897830[SOGoCache]> Using host(s) '127.0.0.1' as server(s)

Well : I think that you can close this issue as there was no more loss of settings appends since the fields modifications and this last problem at 2017-09-06.
Thanks for all.