FTP Connectivity issues while connection is fine

Question

I use the FTP-client FileZilla to transfer my files via FTP. However, recently I noticed many problems with FTP-activities such as listing some directories.

My server is behind VPN, so I doubted the VPN connection at the time of connection issues. However, underneath ping results are during problems with the FTP-client as described above:

64 bytes from 10.8.0.1: icmp_seq=0 ttl=64 time=16.634 ms
64 bytes from 10.8.0.1: icmp_seq=1 ttl=64 time=17.509 ms
64 bytes from 10.8.0.1: icmp_seq=2 ttl=64 time=17.966 ms
64 bytes from 10.8.0.1: icmp_seq=3 ttl=64 time=16.037 ms
64 bytes from 10.8.0.1: icmp_seq=4 ttl=64 time=17.611 ms
64 bytes from 10.8.0.1: icmp_seq=5 ttl=64 time=17.699 ms
64 bytes from 10.8.0.1: icmp_seq=6 ttl=64 time=16.503 ms
64 bytes from 10.8.0.1: icmp_seq=7 ttl=64 time=15.469 ms
64 bytes from 10.8.0.1: icmp_seq=8 ttl=64 time=15.844 ms
64 bytes from 10.8.0.1: icmp_seq=9 ttl=64 time=18.927 ms
64 bytes from 10.8.0.1: icmp_seq=10 ttl=64 time=16.410 ms
64 bytes from 10.8.0.1: icmp_seq=11 ttl=64 time=17.069 ms
64 bytes from 10.8.0.1: icmp_seq=12 ttl=64 time=20.283 ms

Though, FileZilla can't open the directory (with only 2 files in the directory). FileZilla keeps stuck for like 2 minutes on: Listing directory <DIRECTORY>

Then, sometimes the connection times out (while the ping continues)... And some other times, the directory listing succeeds and I can continue with using Filezilla without problems for a few minutes until the same problem arises.

Error:          Connection expired after 20 seconds of inactivity
Error:          Receiving directory listing failed

During the problem I can't upload files neither (they stay in the queue?)

(I use OpenVPN with the Tunnelblick software on my Mac)