It may be your Anti-virus product is scanning any file you open, so temporarilly Thunderbird has no access.

An established method of testing wihout doing anything would be to restart the computer using 'Safe mode with Networking', but if you use Wireless connection then you need to swap aover to using Ethernet cable connection to router whilst testing. Then start Thunderbird. If all works ok then you know it was probably the AV interfering.

However, regardless of whether you can perform that test or not, it is advised that you access your Anti-Virus product and set up an exception to not scan your Thunderbird profile folder. After all, you do not want any AV trying to fix anything in your profile as it's likely to result in loss of email. AV's do not understand that one single mbox text file may contain alot of emails. You do not want any AV doing anything with your profile folders and any file etc it contains.

Paul, did you resolve your issue? And if so, please help us by indicating how that was done?