Right-click the folder in question and choose "Properties / CHMOD" from the context menu (or press F7) and check the rights listed there. You may want to try giving yourself full rights for everything. If you still encouter problems, please post the full connection log from a delete attempt.
I ticked all the boxes but it didn't work.
Here is the log from my delete attempts:
[19:01:22] RMD delete
[19:01:22] 550 delete: File exists
[19:01:22] CWD /delete
[19:01:22] 250 CWD command successful.
[19:01:22] PWD
[19:01:22] 257 "/delete" is current directory.
[19:01:22] PASV
[19:01:22] 227 Entering Passive Mode (62,253,162,29,237,186)
[19:01:22] Opening data connection to 62.253.162.29 Port: 60858
[19:01:22] LIST -aL
[19:01:22] 150 Opening ASCII mode data connection for file list
[19:01:22] 226-Transfer complete.
[19:01:22] 226 Quotas off
[19:01:22] 179 bytes transferred. (637 bytes/s) (281 ms)
[19:01:22] RMD trash
[19:01:22] 550 trash: File exists
[19:01:22] CWD /delete/trash
[19:01:22] 250 CWD command successful.
[19:01:22] PWD
[19:01:22] 257 "/delete/trash" is current directory.
[19:01:22] PASV
[19:01:22] 227 Entering Passive Mode (62,253,162,29,237,188)
[19:01:22] Opening data connection to 62.253.162.29 Port: 60860
[19:01:22] LIST -aL
[19:01:22] 150 Opening ASCII mode data connection for file list
[19:01:22] 226-Transfer complete.
[19:01:22] 226 Quotas off
[19:01:22] 179 bytes transferred. (716 bytes/s) (250 ms)
[19:01:22] RMD trash
[19:01:22] 550 trash: File exists
[19:01:22] CWD /delete/trash/trash
[19:01:22] 250 CWD command successful.
[19:01:22] PWD
[19:01:23] 257 "/delete/trash/trash" is current directory.
[19:01:23] PASV
[19:01:23] 227 Entering Passive Mode (62,253,162,29,237,191)
[19:01:23] Opening data connection to 62.253.162.29 Port: 60863
[19:01:23] LIST -aL
[19:01:23] 150 Opening ASCII mode data connection for file list
[19:01:23] 226-Transfer complete.
[19:01:23] 226 Quotas off
[19:01:23] 117 bytes transferred. (468 bytes/s) (250 ms)
[19:01:23] CWD /delete/trash
[19:01:23] 250 CWD command successful.
[19:01:23] PWD
[19:01:23] 257 "/delete/trash" is current directory.
[19:01:23] RMD trash
[19:01:23] 550 trash: File exists
[19:01:23] CWD /
[19:01:23] 250 CWD command successful.
[19:01:23] PWD
[19:01:23] 257 "/" is current directory.
[19:01:23] PASV
[19:01:23] 227 Entering Passive Mode (62,253,162,29,237,193)
[19:01:23] Opening data connection to 62.253.162.29 Port: 60865
[19:01:23] LIST -aL
[19:01:23] 150 Opening ASCII mode data connection for file list
[19:01:23] 226-Transfer complete.
[19:01:23] 226 Quotas off
[19:01:23] 314 bytes transferred. (1.30 KB/s) (234 ms)