Allocated nnnnn page buffers of xxxxx requested
This means that you don't have enough RAM for the amount of database page cache you configured. You are probably using Classic, which has a separate cache for each connection. The default setting for Classic is only 75 pages, which is usually low, but that's conservative setting assuming a lot of users.
Be careful when calculating the value to put in firebird.conf. It depends on number of users you expect and size of a single page - default is 4kB, but if you have 16kB pages, the cache would consume 4 times more memory with same setting. It is recommended that you go with 256 pages in the cache, which amounts 1MB per connection with 4kB page size. If you have a lot of RAM or low number of connections, increase that to 512, 1024, etc.
Example: 20 users, 4kB page size and cache size of 1024 needs 80MB of RAM.
Please note that if you use Linux, the filesystem already uses all RAM to cache the filesystem (RAM usage on average Linux server never goes below 99%), so increasing the cache is counter-productive as you are creating a duplicate image of what's already in RAM - and thus significantly reduce the total amount of database file kept in memory.