Just commenting for now as i am not too certain about procedure.
The error indicates there's a memory outage (too much used).
This could mean a log is being tried to create, but uses too much from your server.
Now that could indicate that yuor log storage (folder is occupied and holding too much files (which should not), or the time and memory usage for your server is restricted.
Best scenario what could help always; be on local pc install creating a duplicate of your site (15) and update that to 19, and upload the fresh one to your server (including a correct config and dbase of course).
Please wait if unsure for others to reply !!!