[imp] ActiveSync not syncing for one user

[poubeline at free.fr]

Hi all, 

I have 2 users that are currently using the Horde ActiveSync feature and since the upgrade to the latest ActiveSync version, I got the following error for one user in syslog file : 

HORDE: Allowed memory size of 134217728 bytes exhausted (tried to allocate 150994944 bytes) [pid 29577 on line 1090 of "/usr/share/php/Math/BigInteger.php"] 

I have seen a recent log from Luis F e lip e Ma r zaga o about this kind of error but for /us r /s h a r e /p h p/ H o r d e /St r e am.p h p and he found that the user had more than 13k messages in the trash folder. 
I have reviewed the user messages and there are 2 808 files for a total amount of 659,6 Mo. 

I am running the following : 
Horde 5.2.16 
Imp 6.2.20 
Horde Groupware Webmail Edition 5.2.21 
Horde_ActiveSync 2.38.8 

I tried to activate the logging using the /var/log/horde_mobile_access.log file but I cannot get this file to be created. 

Anyway, can anyone help me with issue as I don't have a clue on what's going one for this user ? 

Thank you