I have go through your requirement, There are various reason behind the error.
But we will start with suspicion over server update.
YES, You are right, It cause problem various time when server update
1. Please check that the file path of the image in code, Is it start with the site URL or /sites/default/files/filename.
If it is start with Url then please remove it and start with /sites.. and check
2. What is the php version, if it 5.5 0r 5.6 then please check the curl is installed for this version. if not just try to install it then restart the server and check that problem is solved or not.
3. I hope/sure that the TCPDF library is installed just check once that "/sites/all/libraries/tcpdf/cache" have write permission in server.
If not then please do it clear the caches.
All this are related to server and as you say you will not provide the server access, SO it will better to try the above point and check the problem is solved or not
I am sure if you not made any changes or update in the module then surely it is occur because of server update.
All this are the possible point I am able to suggest for this moment,
If this point are not help or you have already use them, we need to check everything very deeply.
SO, let me know what happen, If this will not helpful then we will work together on it,
All the information form your end is very useful.
Just Ping me once, anytime.
Thank You.