well the allowed size 134217728 bytes equals 128mb which I believe is called memory_limit in php.
So your message tells me that your environment has 128mb set and did not have enough system memory free to allow up to 6695129 possible bytes that ubb is trying to reserve to use for uploading.
So that equates to 6mb which is probably the limit you set in ubb.
I am not sure how to check how much of the memory_limit is being used or what is using it.
But you could try to lower the ubb setting for now to 1mb which would be 1048576 to enter in ubb if you can't increase the memory_limit, At my site memory_limit is set to 256M
